Automatic transmission replacement
 

I'm considering buying an 09 G37 sedan for cheap. The owner says the transmission is toast, it's an automatic. It only has 90k miles. Anyway, I'm looking for the best option to replace/repair this transmission. I see junkyard transmissions with less than a 100,000 miles can be had for just $300. Am I wasting my time looking at these? Can I put a newer transmission, say from a '13, are they more reliable? Should I just consider having the existing one rebuilt? I don't plan on doing any serious mods, maybe CAI and cat-back exhaust. Thanks for your input in advance!