if you are building up a track rat, i recommend getting a cheap stock miata or similar. easier to learn to drive the limit, easier to fix after hitting a wall than a modified vehicle.
Don't forget the big 3 besides just making power and putting a good suspension under it, Tires, Brakes and Safety. I'm sure you well know that these three are important, but it doesn't hurt to mention. Then decide if he's going to be racing in any specific class, or just enjoying a track day. Knowing what your limits are for a class can be a lot more important that finding out what's best and having to work your way backwards.