Check that it will start easily from cold. Insist that you have a dead cold engine for this.
Run it under load full or near full throttle under way for at least 15 minutes and watch for overheating or loss of oil pressure.
Some smoke is OK on start up but very little after that is acceptable.
Take a sample of the engine oil and get it tested for metal debris or oil dilution. [ Not expensive ]
If it starts runs ok runs without losing oil pressure overheating or smoking and the oil analysis guys pass it, well it should be good to go.