ignore any and all "collectable" thoughts, regardless of how logical it may seem. Those who collect over play pinball, are never as happy.
For 20k my opinion is to go for player quality on every game, and play a lot of pinball before spending to much. Most Pinheads do not really know what they like until they have played and understood a lot of games.
You can't go wrong with....something like this.
T2 - 1,200 to 2,500
WOZ - 8,000 ish
XMENLE 5,000 ish
Flash, Gorgar, Mata Hari, Siverball Mania....or some other awesome Solid State game(s) $300 to $2,000
Any 90s Bally Williams- 1,000 to 4,000...FT, RS, TOM....
Any modern Stern- $3,000 to 7,000
Do yourself and the hobby as a whole a favor. Don't get hung up on perfection and collection, stick to fun and function.