what engine are u going to buy? whats wrong with ur current engine ?
How to check ?
- Compression test
- Check the Cams ( use ur finger from the oil cap hole)
- If is posible start it up and listen wat the engine sounds like
- Injector Pulse
What u should change ? Check everything before u buy. Take the water/ oil pump out n check weather is worn. Maybe change new timing belt. Check other belts see any crack or worn.
Hope thats help.