Welcome!
I cant speak to what cars are available out there in California, but I can help with your other questions. Generally it is always cheaper to buy a built car over building yourself. Some love the process of building, so I understand why you might want to do that. There are still some cars that drive to the track and then race, so that is possible under our current rules.
If you are looking for a street car, look for one that is basically stock and currently running. Interior doesn't have to be nice since that will come out anyways. Usual maintenance is nice to see like recent replacements of the water pump, timing and balance shaft belts and rollers, as well as other usual upkeep of the suspension. If you find one with koni or bilstein shocks and upgraded sway bars, thats even better since that is what you will want to do eventually anyways. Stay away from a ran when parked 5 years ago, salvage, or parts car. Those end up being more headache then they are worth once you have to replace everything on it. Be careful when looking at already modded cars. Sometimes the mods end up not being legal for our class if 944Spec is your end state goal.
If the water pump, timing and balance shaft belts and rollers (budget $500) haven't been replaced recently, do that immediately before hitting the track. Also change the engine oil, transmission oil, and bleed the brakes. No need for fancy brake pads on your first event since the car will be much more capable than your abilities. After that, you can look into some more aggressive Hawk brake pads.
Decent running track candidates can be had from between $1-3k depending on how nice of a start you want. If you want to get a car that is ready to go and setup for the track, you can find those starting at $5k.
