If you're that serious about considering it, have an REPUTABLE authorized dealer for the trike conversion go over it to make sure you aren't gonna find a nasty surprise after the purchase (I used to do this with cars & trucks I looked at). Tell the seller you'd like for a dealer to go over the unit, if it's in good shape and has no "hidden" issues, you will pay the fees for them to go over it. IF they find an issue that needs attention, it's up to the seller to pay the shop fees.
It helps keep the seller "honest", and helps ensure you don't end up with a bike that's gonna need a thousand dollars worth of work right after you buy it. That's just my .02