Hey Tyler,
To briefly answer some of those questions...
Yes, I feel it's a good idea to run at least 1 year of Semi-Pro before figuring out if you can move up to Pro. Talent and money will be the biggest factor. New Member classes are completely separate from Semi-Pro, and you will be racing for a chance to qualify for Jackson and that's it. Pros and Semi-Pros races together in New Member classes, but it is a great way to get more runs at the course too.
Semi-Pro I would stick with 800 cc minimum, and if you don't have a couple sleds then run a stock sled so you can race Stock, Improved, and Mod. The more classes, the better.
ISR dictates a lot of stuff as far as sled specs. Tracks can be any length I think, but have to have a certain amount of lug (1" minimum I think). Weight and modifications are determined by what you can do in Improved and Mod. Stock class is basically Stock out of the box. You can make minor changes to clutch components, shock internals, etc... No motor mods, exhaust, or suspension changes unless it's a direct OEM replacement, but again REFERENCE ISR Rulebook if you aren't sure. Follow the additional links in the thread below for more info as well. My advice unless you know someone you can hang with that already has a setup for racing, stick with a stock 800 and get to a couple races to experience it and learn what works and what you can change. I covered a lot of the essentials for safety gear and sled setup earlier in this thread. Study up on that and it should answer a lot of setup questions.
Thread with more RMSHA info links