I would suggest going to a few shows or joining a local club.
Play a bunch and start working on a list of themes and features that you like.
It all starts with that cheap EM, then you want a mutiball machine, then you want multiple modes with a wizard mode.
Before you by an older machine be sure to see if the ramps or other unique bits are available aftermarket.
I waited for years with no avail for a Checkpoint ramp. Had to sell it with the repaired original in it.
Oh, if there is a particilar machine you want there's a way to creep people with one on Pinside. I have done it and recently had it done to me.