You have two "fun" games that compliment each other.

Unfortunately, the two games that IMHO are at the top of my "fun" list (DMD wise) are Scared Stiff and Circus Voltaire. CV are simply too expensive to be a realistic option. Scared Stiff is a lot cheaper, and change hands for $4000 ish. Still expensive, but you already have the two cheaper "fun" games.

Don't limit yourself yourself to Bally / Williams.

Jurassic Park is as good as Adam's Family, if not better. It has the best "toy" on just about any game.

Tales From the Crypt is the "funniest" game I've played, and it is a GOOD playing game.

Maybe consider looking at pinballs that have depreciated over the last 12 months ? Funhouse is a perfect example - $3200 will buy a decent machine.

Stern also have GREAT titles. I was going to suggest Striker Extreme - but you already have a Soccar themed game. T3 is a KILLER game - rulest is not too deep but HARD to complete.

Looking  at a "deeper ruleset" basically rules out anything in the early 80's, but somethink like a Black Knight or a Black Hole are challenging, depending on your mood.