There so many HPDE events out there now that getting out on track has never been easier and as affordable. Typically only wheel-2-wheel racing requires an actual license. Groups like NASA and SCCA require that you go through their HPDE levels before letting you do Time Trials. For most groups putting on driving track days, all you need is a car and a helmet. There are different run groups based on experience; beginner groups will require an instructor to ride with you and some class time during the day. Intermediate groups and above are usually instructor optional and no class time.
Regarding what run group you will start out in, until you are clearly qualified at a higher level, sign up for the beginner group. If they see you are past that level they will sign you off for solo or move you to a higher run group quickly. This past weekend I signed up for B group with Hooked on Driving at Road Atlanta, and they had an instructor ride with me the first session just to make sure that I would not hurt myself or anyone else.
Regarding car prep for the track, I am going to play devil's advocate here. I tracked my 2008 Bullitt Mustang 6 or 7 weekends and NEVER changed my brake fluid. I used the stock pads (though upgraded from the factory) and stock all-season BFG tires. By the time I traded it in, I was ready to upgrade since I was getting too fast for the car's capabilities. I have tracked my 2012 Brembo GT stock for 2 days so far and all I have had to do was wash off a little brake dust. Your Boss is twice the track car my Bullitt was, so do yourself a favor and just drive the car stock your first couple of track days to learn it and your own capabilities, and if tracking a car is something you want to get into. Get yourself a decent helmet (SA 2010 so you can sell it if you need to later), and then wait on buying anything else. The nice thing about Track Attack is that you get to drive the hell out of someone else's car for free. If after Track Attack you want to drive the hell out of your car, then start with upgraded brake pads and DOT 3/4 fluid, and continue with whatever else you wear out on a regular basis. That is where the big money comes in when tracking a car- brake pads, fluid, tires, and other replacement parts. The harder you drive it, the harder it is on your wallet. Part of the reason I can drive my cars stock on the track is because I do not drive the hell out of them. I go fast enough to be fun, and save some of my brakes and tires for next time. This is my only car and I can't afford to replace pads and tires after every track weekend. You might be in a different situation, so you do what you can afford to do. If I had the money I would be out on track once or twice a month.