I'd suggest getting a Gottlieb EM from the 70s, an early solid state from Bally/Stern, a late 80s or 90s Williams/Bally, and a modern Stern. Data East/Sega games can offer a lot of bang for the buck. Buy spreading the budget around the era you get great variety in speed and rules, and variety is key to a collection. It's nice to have a widebody in the collection since the feel is very different.