Alot of problems with optic sensors if anything does not work properly I would get a great lakes board with led readouts to make TS easier. Three large inserts above modes commonly gets wear.
Neutral zone hole usually shows alot of wear as well. Check that both cannons are enabled and test them.

Hope some of this helps I have owned my for 2 years +
Fully working when I got it but took a couple months off and on to get it working perfect

opto errors are super common i.e. does the game load two balls and have a multiball when it's not suposed to? Also check to see if you can hit the delta ramp and make it up to the borg ship. My game seems to have difficulty with that shot (it's steep) even after I rebuilt the flippers. Neutral zone hole takes a beating as well. All problems are fixable it's a great game. Also check for cracked ramps, replacements are available ($300 bucks for a set)

engage.

I think the only reason this game is under 3k is because it's got a lot going on and not easy to work on. Mine is finally fully shopped and fully working and gets as much or more play than TZ, AFM, LOTR WH2O and Creech. Such a good game. Parts are readily available. Don't let Neutral Zone wear deter you, the cliffy will probably cover that. Expect to work on your optos and cannons. It will be worth it. If you get it the shaker is a must have mod for this game.

Its a great game but, you can't be squeamish about working on it. Its a very complex game and things do go wrong with it from time to time. If you not afraid or enjoy working on these, this game is the best.

Optos were cheap and easy for me to replace. Its 85% of the time the emmitter not the receiver. Once I did it work great. Back Drop Target Up release spring needed replacing. Messes the game up if that drop target isnt working right. I had no cannon issues.

6 ball multi ball is crazy fun.

Why did I sell mine?

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oh man.. probably easier to ask what NOT to look for. As others have mentioned, the optos are the biggest problem. Then there are all the ramps, look for chips/missing pieces/cracks (they're expensive to replace). A lot of the chips can be covered with ramp protectors. Look for wear in the neutral zone. Look for a warped Borg ship. Make sure all the ships are there. Make sure the drop target works. Make sure both canons work, and work properly (turns, finds "home", fires balls well, etc). Lift up that PF and check the subways.

And of course, as with all WMS games.. make sure the GI connectors in the head aren't all crispy.

Having just picked one up, I'll list what I had to fix. Most of this is a repeat of what has been said already.

-The stand up target's actuator was broken
-Bad opto receiver for the left popper
-Divots in the ball trough that had to be filed out to keep balls from hanging up
-Left popper wouldn't reliably kick out balls when other balls were locked, needed adjustment
-Balls would get hung up when rolling from the underplayfield ramp into the left gun's popper
-Typical burned GI connectors
-One GI string was flaky, ended up being baked connections on the PDB from heat
-Had to resolder bulb holders on the large PCB for the ship's shield inserts
-Going to rebuild the cannon wiring harnesses as they have been hacked in the past
-Right flipper opto interrupter was wimpy and wouldn't fully block the beam, had to be replaced

If I can think of anything else, I'll add it later.

Edit, just thought of more:
-Cracked subway ramp at the Start Mission hole. I repaired it and ordered the protectors from Mantis Amusements
-Broken Start Mission scoop. Had it welded at a local shop.
-Broken left flipper base plate
-Broken pop bumper rod

You should buy it, because then I won't be able to. Denver is a really long drive.

Congrats, that is a fun pin.