I honestly wouldn't spend anything more than $10k max. I bought my car which was factory manual, gts-4, full exhaust, aftermarket turbo with about 112 thousand Ks. It cost me $9k with rego and roadworthy.
Just check over the car to make sure everything checks out ok. If you wish to pay more for a better condition car then go straight ahead. I would, but didn't have to.
Good luck.