buying a used excavator - TractorByNet

Im looking to buy a used excavator to finish some yard work. I had my 50 acres selectivly cut (pine and hemlock) last year and Id like to pull the stumps and clean up the slash. One area will be turned into a field, one area cleaned up for apple trees and the major project of building my MX track.
The machine Im looking at is owned by the engineer that sits across from me at work. Im going to look at it this week. He maintains it by the book, religously. From greasing, oil & filter changes, and track adjustments. Previous owner used it in a screening operation, very limited movement. Under carriage is probably about 60%. Only down side is there is no thumb.
So what should I be looking for? If he's taken care of it that well, you're already 1 step up.
I was in the same situation last year, 62 acres that had been selectively logged before I bought it, ala stumpville. I bought a dozer and started the process for clearing trails and a homesite, but finally realized I needed an excavator for all the stumps.
What I did for the first one I looked at was have a nearby equipment dealer send out a tech/mech to go over it 100%. I was with him so I could see all he looked at. The cylinders were seeping, the engine had blowby, the machine couldn't even turn itself on the tracks at low rpm.....needless to say I passed on it, and learned a bit about what to look for.
Undercarriage - obvious stuff...sprockets, rollers, bushings, etc. Have someone drive it while you look at the tracks and running gear.
Engine - blowby, black/blue smoke, obvious engine stuff.
Cylinders - seepage......will need repacking.
Booms - check very closely for cracks. The machine I bought had a crack in the upper boom, but I knew about it and it's already fixed. Also pins for play.

I'm by no means a mechanic, and am sure someone else could give you much more detailed info to look for, this was just my limited experience. I think I paid the "real" mechanic around $150 for the inspection, money well spent.

Is it a mini ex or larger? Steel/rubber tracks?
I ended up with a Cat 312, I'm glad I bought this size to get these stumps out, dig the pond, foundation, etc.
Good luck!