Is it cheaper to recondition a working engine rather than waiting for it to die and then doing it?

Would it be a case of new piston rings, or would it need doing properly with new internals too? I'm happy for the engine to live around the 320bhp mark so I won't be wanting the expense of stronger upgrades, just stock.

I ask because i'm thinking of doing the whole turbo/exhaust combo on an 180k 855, but don't want to kill the engine due to it being tired (or wasting the upgrades on an engine not willing for that power)

A few of us have had engine problems, so I'd love any opinions on the matter.