theres no point delaying your first event. go for it. Even if you dont have all your go fast part on, or even good tires. As everyone else says, you need seat time. even seat time in a buick (or other unperformance car) is worth it.
and for you other question about allowed mods, read up on the rules, but you have a carte blanche in SM, add whatever you want, you just cant remove the interior. but as for tires, the best you can do is put fender flares on and run a really wide wheel and some super wide sticky tires. im thinking the 275 hoosiers.
for auto-x: '04 Liquid Grey ZX3
for street: '02 Infrared SVT