Okay, let me see if I can help out here.
The car you're looking at, a 99 Trans Am is a very nice car, especially the WS6 version. However, if you're looking for the same performance with a slightly better cost, you MAY want to look at a Formula as well.
A Formula is almost IDENTICAL to a Trans Am, aside from a few exterior panels. They both share the 5.7L LS1 V8, and both offer a 4 speed automatic or a 6 speed manual gearbox. Formulas do not come with as many STANDARD options as a T/A, but other then that, they are the same car.
You can pick up a '98 Formula for right around 16-20 in GREAT condition.
Also, check out the 'Classified' section at:
www.LS1Tech.com and
www.LS1.com. A lot of people that have modified their cars sell them on those sites and you may be able to buy one already modified (if that is what you're looking for)
Good Luck. Any other questions, feel free to ask.