Hello fiction,
I just did this whole process about 2 months ago. There is a whole thread I read through before I purchased here:
GTRLIFE NISSAN GTR (R35) FAQ shows all the MY differences and lots of great info.
The time it takes to find a good car is the longest part. There are some lemons out there for sure. In todays market with your budget your PROBABLY looking at a CBA (09-11) GTRs with low-medium miles or an older DBA 12+ with medium to high miles. Lots of smarter guys on here than me, but apparently at the FBO level (if you plan to mod), they all make roughly the same power when custom tuned +/- different MY specific variables. If you do go CBA, the trans needs some TSBs done if they haven't been done (make sure to get a history report). Just did the clips on my car and changed all of my fluids: diffs, trans, and motor-pretty straightforward.
Also, get a PPI or (Pre Purchase inspection), very important to do. It will tell you if the car was launched, how many times launched, and list the temps the oil was in (higher temps means more abuse). Finally, this should go without saying, but test drive it as well.
Good Luck in your search!