To answer the OP's question, my preference would either be a brand new engine, if the budget allows, or, if less money is available, an engine in mid life, which has run regularly over a long period (NOT a hangar queen, and not damaged), which is then at least top overhauled before your installation, or better a top overhaul and the case split for inspection. And, in either case all accessories overhauled. Then you will know what you started with, and have confidence. In your new labour of love plane, a low cost unknown engine is not what you want!
Avoid Lycomings which have sat not run for periods of months, unless they were properly preserved.
I say this having worked in an engine overhaul shop for years, and owning both a Continental (O-200) and Lycoming (O-360).
The very coarse fixed pitch prop is fine on the RV, but you will sacrifice some takeoff and climb performance. Fixed pitch is always a compromise. I have flown MTV CS propellers on a number of O-360 powered aircraft (Lake Amphibian, Twin Comanche, DA-42-360, and my Teal Amphibian), and very much like them. I did the back to front testing of the Twin Comanche with the MTV's having also just tested the two blade CS Hartzells. There was a noticeable improvement in climb performance.
For taildragger aircraft, there is a added (though hopefully little used) benefit to having the MTV propeller: In the worst case, if you accidentally prop strike the aircraft, the blades will take the hit, and your hub and engine will survive undamaged. They will still need inspection, but will most likely survive. If you ground strike a metal prop, you're replacing the entire prop, and probably some of the engine.