Well, I am a big proponent for brakes, so PF01 pads would be a nice addition. Next, tires and wheels is what your car rides on, and a square set up with 18" tires would also be cost justified. I recommend R compound - tire brand you pick em. Seats and restraint system always a good add on depending upon how deep you care to get into the sport. Now, doubt you can do all of this on your budget, note tires and rims will be costly, as will seats and belts.
vz
ps - Just thought - given all the discussion on a recent subject this forum, a large roll of painters blue tape is a must to cover you license plate..
vz