I just got big into it this season. The Mazda3 is a terrific platform. The 2nd gen that I'm driving might be considered a little heavy...but the first gens are great.
The sticky is a little old in defining what the "street category" is. Best bet Google the SCCA rule book and read up on that.
What I've been told is just focus on your own driving ability primarily, and the first hard mod would be tires, if you are so inclined.
It's definitely easy to get hooked. I ponied up to join the local club, and the events haven't been terribly expensive. Check around for your local SCCA chapter. I think I've seen some driving in stadium parking lots (meadowlands?) It also allowed me to join the mazdaspeed motorsports program, which is totally worth it for parts discounts if you are a DIYer.
Make friends! Have fun! If there's a novice program or something that gets you some instructor availability, jump on it. The external critiques are invaluable and seat time is always good.
2011 Graphite Mica Mazda3 sSport 6-spd MTX