Is it Hard to get an Engine Rebuilt in a Used Vehical (RETYPED)?

I'd like to buy this Vehical of my brother, Mazda B2200. 380,000km on it. It runs great, it needs new valves apperently soon MAYBE.. and it has new brakes/pads/carb/o2 censor. Runs great for an older vehical but im scared the engine might die on me in a year or so. What would it cost to get an New engine in it/ overhaul i guess it means and new valves replaced? please give me an estimate i dont want to phone a shop cause i dont have the vehical yet but might in a few months thanks!