Any GT-R now with the age of them will cost you money in repairs, minor things like clutches and turbo's are in thousands and even to the extend half or equal to what the purpose price was after a couple of years. GTST or RB25 powered skylines are great value with little cost's in repairs, especially now with some R34 GTT's becoming great value. Owning both the GT-R is amazing to drive but has cost plenty to maintain, while R33's provide reasonable performance without the stress of shitting your pants if you break something and are good value and making 200-300 safe hp. End of the day, you get what you pay for. Finding a cheap bargin might end up costing you in the long run, looking around and paying alittle extra for something that has had the general repairs and upgrades done might save you down the track. So $20,000 with money ready for insurance and repairs would get me a R32 GT-R, $20,000 running on a budget would get me a R34 GTT or R33 S2 GTST which has had a few bucks put into it.
Also, get a mechanical check up if you have found something that has caught your eye. These include the compression and leakdown tests while getting a overall checkup of the vehicle and what sort of wear it has had.