I've never purchased pieces of ride hardware. In fact I own one piece, a side friction wheel off a coaster that was located at American Adventure. But I got that for free, I live really close to the site and I found it just lying there on the ground, right on the footprint of where the coaster used to stand. (Buffalo if anybody is interested.)
I used to use it as a pen holder on my desk, being a side friction wheel it's taller and slimmer so perfect for that.
Anyway, I digress, I've never bought hardware, but I would 100% try and bag me a piece of Nemesis. Would almost complete my collection so to speak, American Adventure and Nemesis are what started it all for me.