At the risk of giving out information which may be wrong and put people in danger, I'll wait for an engineer to advise on what to look for!
To answer the question about paying for the test flight, though, I would always offer to pay half. Bear in mind that, unless the pilot has a CPL, you're not allowed to pay more than half (assuming there are two of you on board.) If my offer is declined, then that might tell me something about how friendly the group is.
FFF
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