High miles would scare me outside of warranty. Maybe not so much with a manual.
Some bargain e92s around at the moment. Especially late DCT's.
Any cars which are expensive brand new, that have high miles, are over 5 years old and are out of warranty would worry me. But then I'm a wimp!
For 20k I wouldn't know what to look for. Think I would keep the 350z!!! Unless you find 30/40k then I would change it for something worth the extra money.
It's difficult because you get a lot of car for very little money in a 350z. So unless you're a brand wh0re, and fancy a BM or something similar. Keep it and book a holiday instead!!
Good luck!