Just started a restoration of one myself, are you looking at the one in Washington or New Jersey? I assume NOT the one in Indianapolis, as that one looks very trashed.
Look for the backbox to be structurally sound, and the backglass in decent shape. Playfield condition is up to you of course, generally they are worn to some degree... after all they ARE 30 years old now.
The big issues to look for are scorched components on the power supply, though the GI connectors will almost always be burned to a degree (Bottom right on the power supply). Only use #47 replacement bulbs, instead of the original #44's, to help take the load off the GI circuit.
Check that there is no battery corrosion on the main board, it's a big plus if the batteries have already been relocated. There are some recommended updates to to do the whole power supply system, regardless of how well it performs, these are well documented on pinrepair.com (Black Knight is a System 7 game). Definitely read through this, as some are critical to do.
As far as playstyle, it's a fast game and target intensive. Sure there are three "ramps", but it's definitely different than what you've got in your collection already. Think of it more like a hybrid of 70's drop-target mania, with a lot more speed, early 80's arcade type sounds, and a brutal multiball.
Personally, I think the playfield (when in good shape) is one of the best looking of any era. But mine isn't playable just yet.