Hello DracoreX and welcome to the forum. I took the plunge into hot keeping about a year and a half ago, after 7+ years of non-venomous snake keeping. I can tell you that there is no snake that will completely prepare you for the deadly responsibility of keeping venomous snakes- they are in a class all their own and unpredictable. However, there are certain non-ven species that can be moderately decent practice, depending on what type of hot you want to own.
First off, I commend you for asking questions and looking for training and practice before buying a hot....that is the proper way to go about it and I love seeing people ask questions and think long and hard before diving in. It's a big risk and a big responsibility, and you can never be careful enough. A mentor would be your best bet; however, it will likely be hard to find one until you turn 18. You are still a minor at 17, and therefore very few keepers will be likely to take the risk of you getting hurt under their watch. However, you should ask around at local reptile expos and be active on venomous forums to look for people who might help you.
My mentor lives about an hour from me, and I get to work with his animals once or twice a month. I have my own, but I am grateful for the extra practice and training I am continuously receiving with him. Working with a seasoned, experienced keeper can really help you highlight your areas of weakness and (relatively) safely work to strengthen them. I am not sure which state you live in, so I don't know where to point you other than forums to look for a mentor.
As for licensing/permits, it is different in every state. Some states, like PA where I live, do not require ANY license or permits, and pretty much anyone can go out and buy a venomous snake. I would much prefer they require licenses and some extent of training to weed out irresponsible people who just want a hot because it's "cool."
Other states ban them completely, and others require various licenses/permits or a certain number of hours of training under a licensed mentor (Florida requires 1000 hours of work under a mentor, with detailed records kept).
You should be able to find information on your state, and local, regulations on the Fish and Wildlife website, and your city's municipal website under the codes section. Most rural small towns do not care, but you must be sure. You don't want to end up with hundreds or thousands of dollars of fines later on.
Now, moving on to good "practice" species. Well, as I said, there isn't any non-ven that will truly mimic what it's like to have a venomous snake, but there are some species that can help you hone the handling skills you will need with various venomous snakes. Which species depends on which venomous species you want to keep.
For people who desire elapids (cobras, mambas), I would suggest mean, wild-caught red tailed green rat snake, or even better yet, a Spilotes pullatus (tiger rat snake). I have a LTC Spilotes that is convinced she is a cobra. She will flatten her neck, rear up, and she is lightning fast and wants to bite. Learning to hook and tail her without being nailed got me to where my cobra isn't all that bad to work with. I would say the Spilotes is faster and more aggressive in many ways, although a bite from her is no biggie and a bite from Shelby would be the end.
If you are looking to get into arboreal viperids, a green tree python or Amazon tree boa is a good practice species. The ATB will be smaller and quicker, and can be hardier in captivity, but both are good. I'd kept GTPs, ATBs, and ETBs for 6 years before I got a bush viper and an eyelash viper, so the vipers were almost identical in movement/behavior. You will have to keep in mind, though, that an arboreal viper can strike a longer proportion of its body than a python/boa can. For example, my bush vipers routinely throw their entire body at me, holding onto a branch only by the last few centimeters of tail. A GTP or ATB would usually only strike 1/3 to 1/2 its body length....so you have to be aware of your vipers' abilities and make sure you're out of strike range. You cannot tail arboreal vipers- they are far too agile and can easily whip back around to nail you....so you must get really good with one hook and two hooks (one in each hand).
Rattlesnakes and terrestrial vipers can be as varied as the weather....some are ambush hunters, and they tend to slow and still, but can explode with speed and agility when they are hungry or angry (rhinos, Gaboons, and the like). For these, a foul-tempered blood python would probably be the best practice, although it will not really be like these guys. For rattlers, copperheads, and the like, keeping a variety of other quick, ill-tempered terrestrial snakes can help you (most boas, pythons, colubrids will help you pick up hooking and dodging skills, but will not really behave like a hot).
False water cobras are rear-fanged venomous. They are not terribly dangerous to humans, although a severe bite can cause swelling, blistering and flu-like symptoms for several weeks. Most bites result only in a slightly excessive bleeding time and mild swelling. Rear-fanged snakes have poor venom delivery and they need to hold on and chew the venom into you. That being said, I've had FWCs, and they were nothing like my cobra. They might be fair practice for rattlers or the like, but my FWC was docile and generally not very quick.
As for providing venom for industry, a private keeper cannot really do it. There needs to be USDA and FWS licensing of your facility and you would need to keep hundreds of snakes to make it worth your time and money. For most antivenins, you need dozens and dozens of venom samples to produce a single vial of antivenin. Also, you stand a MUCH greater risk of being bitten when you are milking venomous snakes, so I would not think that planning to provide venom samples should be something that you even consider. If you are set on it, you would need to look into going to work for a venom bank. That's pretty much the only way I see it possible.
Now, on to good "first hots.".....there aren't any. Honest. Now, that doesn't mean that I don't think people should keep hots, just that I don't feel that there are any that are that much less dangerous than others. There are certain things I would never suggest that anyone own as a first venomous snake- any of the Naja species, Western diamondback rattler (too aggressive), Eastern diamondback rattler (too big), and Gaboon or rhino vipers (too unpredictable).
I own a pair of juvenile Southern copperheads, and every single time that I have to clean their cages, the little buggers give me a run for my money. They are squirmy and lightning fast and do not hook well at all. Coppers are very nervous, high-strung snakes, so they can be a handful. Many people downplay the seriousness of copperhead evenomation; in the US, bites from them have significantly greater morbidity than rattler bites, because hospitals are reluctant to give antivenin. "Oh, it's only a copperhead bite." So people end up with permanent or very long-lasting nerve and tissue damage. Don't take them lightly.
Some of the rattler species are less toxic and tend to be more docile....things like speckled rattlers and Panamint rattlers would be ok first rattlers. They probably wouldn't be a horrible choice for first hots, as long as you had solid training. I personally think that arboreal viperids are probably the easiest transition for most people. They tend to hook very nicely, as they are instinctual tree dwellers, and a hook to them is like a branch. They can be hook climbers though, so you have to be ready with a second hook to keep them away from you. Eyelash vipers are the most readily available and well-studied, and bites can be treated with the same antivenin used for native US crotalids. Eyelash bites are not pleasant, and can result in the loss of a digit, but they are not as likely to kill you as a diamondback bite. My fiance bought a baby eyelash viper, and that was the first hot in the house. Shortly after, I bought a horned bush viper. She was only about 10 inches long, but utterly vile; she was great. I love bush vipers, but there is no antivenin and they can be fatal.
Well, I'll keep on rambling all day if I don't stop myself, so basically good luck; your attitude and approach are right. Take it slow, look into some of the "practice" species I suggested, look for a mentor and start researching any hots you might be interested in keeping.
The University of Adelaide has a great database to search habitats, venom composition and bite protocols.
WCH Clinical Toxinology Resources.
Any more questions, feel free to ask. You can also PM me anytime and I'll help you where I can.