To be honest, I would source your parts from a molder. Buying a machine used (blindly) and not knowing the technicals of molding will cost well more than just the machine cost.
If you're dead-set on doing it yourself, look for good name brand machines. We've been replacing our older machines with newer Nissei machines. My suggestion is to look at Van Dorn machines. They're a dime a dozen, parts are cheap, and they're easy to work on. Arburg (from experience) is way expensive for parts if you need to fix things. We have a 520c that we bought new in 2004 and we've spent more on parts in the last few years.

Just my $.02.