I *just* got my first FW a couple days ago, so my knowledge is limited and 99.9% comes from what I've learned here and on Nova Montgomery's site.
One tip I picked up from her site is to check the brushes in the motor. If they don't come out with the spring and you can't fish them out with it, apparently that's an indication of soot build up, which happens when the motor is run dry. This (according to Nova's video) means the motor is on its way to needing to be replaced. If nothing else it can be a sign that the owner didn't really do the proper maintenance on the machine.
I have been watching a lot of her videos - you might want to watch some of her maintenance videos, she has some really great info and might help you figure out what is a deal breaker and what you can take care of on your own.
Hahah, and I just noticed how I worded my first sentence there. "I just got my first FW..." Oh man. I'm in deep doo-doo. I thought I only wanted the one but apparently the collector part of my brain is already scheming to get another one, LOL.