No need to replace the pistons unless they are damaged. You just need new rings and bearings (Crank journal and big end), new oil seals for either end of the crank and gaskets all round. Do the valve stem seals in the head while your at it too, plus the oil pump and water pump. I've only done an SR20, and I would put aside $3000 for a good rebuild with OEM replacement parts (SRs have a chain which costs a lot to replace). Expect more if there are any unexpected discoveries. Don't really know the costs for an RB though, could be different.