If you're going to have to replace the back glass with an after market glass what will that cost? You HAVE TO HAVE the back glass to enjoy the game.
"plunger"?? - sound like you're describing an outhole kicker. Could be dirt in contacts, broken wire or an open coil or something else.
Score display - they can be found pretty inexpensive - either an old original display or a new after market display - IF the display is bad and NOT a more serious problem with a circuit board - games that old can have all kinds of board and wiring harness/terminal housing and pin oxidation/corrosion problems.
The four important value components of a pin game are:
Playfield - is it visibly and badly worn or in pretty good cosmetic shape?
Back Glass - is it intact or flaking or otherwise bad (this game doesn't have one) Not Good
Cabinet Condition - self explanatory.
Does Everything Work Properly?
If you're not electronically gifter it can cost several hundred dollars to have someone fix the problems with game play and the display. Are you handy with a meter? Soldering iron? PCBs? Reading a schematic?
Parts, a new back glass, repair/labor, cleaning with new bulbs and rubbers and adjustment could cost hundreds of dollars on top of the base price.
It could still be a good deal but for a 1980 game maybe not such a good deal.
Check out craigslist and other local advertising venues for games. If you can afford it, I would prefer to buy a game in good physical condition and good working order than a "project" game that needs a lot of fixing up.
I'd suggest you try to find a "everything works" pin for your first home pin.
I'm in N. Florida and often see pins for sale for just a few hundred bucks - A year or so ago I bought a nice 1992 Williams DMD game for under $500 and it took about two hours for me to correct what needed correcting.
The way you describe the game I'd think twice.