There are 4 bumpers in the scoring rings. Three are in the 10 point ring and one in the 20 point ring. The stock ones on my machines were worn down to the metal fasteners and were used for a long while like that. The metal fasteners chipped away at the wood composite balls bad enough to affect the rolling of the balls. A set of used balls is fairly hard to come by, unless if you're willing to pay an arm and a leg for a NOS set on ebay, and even then used sets go for about $10 a ball. If the bumpers are worn badly, that could be a negotiating point. I don't know if you can get original replacements, but you can buy common furniture parts and hardware very cheaply and have an acceptable substitute.
On the ball subject, I prefer the wood composite balls. But there are also nylon balls. Worn bumpers won't affect nylon balls nearly as bad as wood composite, but there's just something about the feel of the wood composite balls during play...
There are three sand bags in the scoring rings as well. They are in the 50, 40, and 30 point rings. They are there to help prevent the ball from bouncing out of those holes. Nearly all mine are worn through and have no sand. I haven't really had a problem with balls bouncing out though, but it is something else to look at.
I don't think the lane carpet is original on that machine. The original lane carpet was a pinkish red cork. The original material is very expensive. Might be another negotiating point. It looks like a model H machine converted for an amusement place, the marquee and beacon are different as well, but it looks like it has the bones of a model H.
It's missing a front net, not a big deal if this is for personal use. But if it's to go on location, it will be extremely easy to cheat.
The ticket dispenser isn't that important if this is for personal use, but definitely needed if you're going to be putting it on location so you can hand out prizes. If it is for personal use and you have the ticket dispenser enabled via the dip switches, once the tickets run out, the display will say "HELP" and you won't e able to play until either the tickets are refilled, or the dispenser is disabled.
Check the back legs to see if the levelling feet are present in the legs. One of mine is missing one, I haven't done any hunting yet, so I don't know how hard they'll be to find, but I think I might be able to find them in a home improvement store.
Have him pull the score ring out while you're there. Check the switch actuator wires. They'll probably be bent (adjusted) in various ways, but make sure the actuator wires are stiff. If they have been changed out in the past, there might be softer actuators which will require constant adjustment or replacement. New switches are also expensive in the states. But can be found for a reasonable price at arcadeshop.de.
I paid $100 for one of mine and $50 for the other from the same guy. Both were "non-working" but I took parts from one for the other to make a perfectly functional one. Spent about $150 in parts for the other one (including another set of wood composite balls) to get it up to snuff.
$800? If you feel comfortable spending that, sure. But it's not original, if that matters to you. Try to talk him down to around $400 or $500.
I've attached a couple pics of mine for comparison. On the right side, yours is missing a view window for the balls, that piece was might have been replaced with a left side piece, or maybe it's different from the factory. You can see the sand bags and bumpers in the second pic. Also, you might encounter some of the angle pieces being broken that hold the score rings.