Bit of an ambitious project, but hey great idea all the same. Will certainly look forward to seeing it come together.
Pinballs are certainly cheaper at the moment, but getting hold of a "good runner" even a beat up one in that price range might be hard.
You could always increase the budget to maybe $1200 and then sell off the playfield and the backglass for maybe $200 each and that will bring you back to your original price range.
What sort of theme are you thinking??