Ok, just looked at an Onan today (thanks here to whomever, Onan is what I want!!). Brother is buying it but I'm in charge because I 'know more about this stuff' which is basicly very little.
Looked at one that I knew I shouldn't have but did anyway, and I shouldn't have even looked. But here is what I gather:
-Compression check absolutely, no purchase without it. Check the plugs also (Greyish?)
-Load test if possible? (but these are not always ready for that, just have the 3 wires- how to hook them up quickly???)
-Make sure the thing is stored indoors (This one was outside, no cover that I could see, not kidding)
-Make sure the seller is not a complete idiot, this one sounded 'good' over the phone, but a complete.....you know.
-The oil/fluid catch pan had some old oil, is this normal? It was completely covered in crap.
That seems like minimum without freaking out the seller, anything else? Compression check, load test. What size plug on the compression checker?
The guy I went to was a complete moron, the thing was outside,(now its snowing) he kept whinning about how he had to sell it, yet it was all covered in backroad dust, years worth, and it didn't start when I looked at it today, (he thought it was a fuel-pump) I was pretty pissed off about wasting time and gas. But learned a bit.