Its very difficult to put a price on an engine rebuild, especially if you don't know what is wrong with it in the first place. Replacing the rings is not a rebuild. Its a repair or a freshen up.
If you open up the engine and find a scored bore, you could be lucky with a hone to repair the damage and a re-ring. Otherwise you are in for a rebore. The rebore in itself is not that expensive. The new pistons are.
Regrinding the crank is not a mojor issue either if it needs one, nor is replacing the oil pump. But then if you start going into head work, you could be facing a bill in excess of 400 quid for a recon.
My advice is to open up the engine and find out what the problem is, then address the problem. Whilst the engine is open, a new oil pump is a good idea. If you start going too deep into the engine and wanting to repair everything you see, you will end up with an empty wallet. A re-build is not what is called for, but a repair, dependant on what is wrong, should see the engine through to long after the car is dead.
