I've raced RD400's and RZ350's for years. There are still plenty of parts available and about 4 decent Forums to go to for advice. As far as picking up an RZ...unfortunetly it looks like the boat sailed on cheap RZ's about 10 years ago. I saw an RZ500 for sale the other day with 35K miles and been sitting for 10 years and the auction is up to $5500. Just stupid money considering the work needed to get it road worthy again.
The best deals are found if you know a local person that has one and doesn't really feel like messing with it. I know a guy that picked one up last month for $500 but it was in 300 pieces. The plus is that they are easy to work on and like I said, parts are easy to get.
My race bike is currently in 300 pieces.....I'm putting a 2004 R6 front end on it and an FZ600 swingarm (a common swap) so I can mount modern rubber. Then it's time for engine work! Looking at a 421cc kit.
If you look hard there are still a few deals to be had on RD350's and 400's. Or go the smaller route to start.....plenty of 125's, 200's and 250's out there. I actually have 3 RD125's, also in 100's of pieces.
I guess I didn't really answer your question. The main thing I look for is condition of the cylinder and play of the connecting rods. It is so easy to remove the head and do a visual inspection. If the seller refuses, that is a red flag. Everything else about them is so simple that I don't worry. A monkey with a wrench can fix them. Now tuning them properly....that is a black art!
It's an addiction......you can't have just one 2 stroke!
My RD400....took it down to the frame and rebuilt from scratch..
And I like a 4 stroke every now and then too! My FZR400 that I recently got rid of...