The cheapest most airworthy or NEW
Going fixed is a good idea, especially since the Sensenich line of metal props has come out specifically for the RVs in the last year or so.
You may resign your self to paying for a new engine, which will provide real value. Since ECI and Superior have came out with their clones, the price of a new engine are really a bargain. Buying a used engine can be hit or miss, the miss part may cost many thousand and you will end up with an overhauled engine that cost as much as if you went new from the get go.** However if you can find a running engine and FWF say off a RV that is upgrading to a 180HP that can represent a real bargain. If you by a core and it is good it will cost $8,000-$10,000 to rebuild. If you pay $5,000 for the core you are up to $15,000. If you have any hidden surprises it may end up costing as much or more than a new engine, and you still have a used engine. If you buy used shoot for a running engine that will be able to be used as is with out rebuild, other wise going new is not a bad deal.
Mattituck and Aero-sport are kind of the king of the hill's in providing and catering to experimental builders. There are many great shops, these are just two that come to mind I know have a great reputation. Also you can build your own engine out of new parts and save some money, however I think the value of having a pro shop build, test and warrantee the engine is worth the small additional cost (about $2,000?). In the end a new engine will give you 10 to 20 years of trouble free flying and if youn sell it will add to the value. (As far as buying a fixed pitch only or C/S prop engine, consider buying a C/S prop engine for future upgrade potential, just an idea.)
Good Luck G
**I bought a used O-360A1A for cheap and got lucky and rebuilt it myself. I lucked out and have a zerotime 180hp engine for less than $12,000 so uesd can be a savings but will take more effort.