Do the pinballs stay in or often bounce out of the Merlin hole is a common concern. In many originals they often do and it wouldn't be a deal breaker for me and you can put things around the hole to help that. I believe there's a dealer or two that make some things to help so the ball often does stay in more basically some stuff that's sticky on one side you attach. Although some here have suggested back handing that shot helps if you prefer just to increase skill and make no adjustments. On some machine the balls can bounce out of the castle and not register an important shot including destroying a castle. This is usually just a minor adjustment. When I bought mine the castle gate appeared to go down before I had earned it to do that sometimes. I finally figured out the optos near the gate needed some adjusting. On some games over time some of the parts of the castles don't stand up straight and lean a lot. Again none of these individually are horrible and there are many good threads in fixing these and others concerns.

Then of course there's the usual playfield, artwork, and cabinet condition that are considerations in any pin you buy. Also switches that need adjusting and other things but above I tried to include what I know of unique things to look out for with MM. I hope this works out for you.

If i was in front of The machine, i would hit test buttom. And then i would check all castle area ( castle exploding animation, bridge and gate) and troll in test or game. There is also a usual playfield wear above flippers.
Excellent choice, best ever.

Specific things to check where I can think of - there's not too much as MM doesn't use too many special/fragile assemblies.
Drawbridge and trolls is about all there is.. check further for broken plastics, do the switch test, ..

Playfield / insert damage (above flippers).
Playfield damage in left lane where the moat kicks out, on top right lane above rollovers where pinball lands from launch ramp, and in front of castle (ball lock)..

Broken plastics (archers above slingshots are often broken/missing, and the dragon wings)

Do trolls work good, do they come and stay up (selftest for this), and register well.

Does the drawbridge work well, goes up and down (sometimes the gear is missing some teeth and then it'll fall down for the last part) and doesn't make too much noise.

Check to see if the green moat subway plastic is cracked. It breaks where the ball drops in from shooting the castle wall flap. Not a cheap piece.

Not a deal breaker, but the Troll carriages have a tendency to fracture the welds at the boss where that the 1/2 rod attaches to. If so, it's an easy weld. I can fix them for you if need be. If the Trolls are slow going up and down, good chance the roll pin that holds the 2 bars together, in the coil sleeve, is working it's way out. (Yup, that long rod is 2 put together). Either needs to be taped back in, or eliminate the roll pin, weld and finish it. That's what I did on mine.

Great Machine, you will enjoy the hell out of it.

Good luck!!

Just out of curiosity, is it the one up at Leisure Time Amusement in Winston you'll be looking at? If so, I've seen and played that one a decent amount of times and could tell you about it.