There's no point in replacing pistons only. If they are worn out of spec or failed then you will need a bore and hone anyway at the least. You also will need to ensure correct piston to bore clearance then as super-da9 says the honing is important for bedding in the rings. While it's out you would at the minimum replace the bearings as well, clearances have to be checked at the same time the crank may need polishing or linishing of the journals.

There's many things to check when the block is apart, it really needs full inspection before reassembling anyway. The cost will always be more than you think, if you do all the removal, tear down and reassembly yourself I'd guess prices around the 3k mark once you factor in all the consumables/gaskets/seals etc that would be needed. That is for a cheap build too without any fancy brand name parts bumping up the price.