Of what you have listed, I would recommend a Stern TMNT Pro as your first pin. It fits in your budget and you can get it brand new and delivered for around $7000 or used for about $6500. One of the hardest things it was for me to realize starting out in this hobby is how easy it is to buy/sell/trade these things, as long as you don’t mind moving heavy furniture. There are now 20ish Stern Spike 2 games, that is Stern games with an LCD that you could easily trade a TMNT Pro for +/- $500 cash. Get tired of TMNT Pro after a year? Trade it for Star Wars, Black Knight Swords of Rage, or Avengers for about even money. Add $500 for Deadpool or Jurassic Park. Ive bought/sold/traded 40 pins in the past 4 years. Yes you read that right. I’m in Maryland, but still.
The Simpsons Pinball Party is another excellent pin and would be my number two for you. Stern’s from this vintage (early 2000s) are generally rock solid reliable. I had a TSPP and it was great. If you can find one in your budget, go for it. Note that your can usually find these in non perfect cosmetic shape (wear at the itchy and scratchy hole, worn out shooter land, white area around buttons) for a significant discount. If you don’t care about this sort of thing you can get a deal.
I’ve never played Wonka or DE turtles so can’t speak to.
DE Simpsons is a great game, but in a one pin collection it may get old… especially since you can just shoot the ramp repeatedly to blow up the score.
STTNG is my absolute favorite all time game. Great theme, assets, music, gameplay, lots of modes, and is still relatively cheap for what it is. HOWEVER I absolutely do not recommend it for first time pin buyers. It is 100 pounds heavier than a normal pinball machine, and you need to learn to move normal sized pins first. The optos and mechs in there are notoriously difficult to maintain, especially the cannon looms that shoot the ball. Great game, but not recommended for first timers.
As to gameplay, you will never beat TSPP, and there is a lot of depth and replayability to it. TMNT is another deep game with plenty to do to keep you busy.
If you are worried about parts availability, don’t. With the exception of Stern Node boards which are in short supply now but still being made (Modern Supply Chain Crises) you can get parts for everything mentioned. The older ones are more likely to break, but easier to repair. For the old games, if a component on a circuit board breaks you can get a new surface mounted board to fix it. If you ask nice, there are threads dedicated to repair and many a first time pin buyer have been coached through repair with lots of pictures (me). If that is beyond you, you can now literally buy a new sound board, CPU card, and solenoid driver board for every pin mentioned. Yes… even the 30 year old ones.
Good luck and let us know what you decide!