in my build i use wiseco piston, got them from the states with eagle rods, went for 86mm ones and had the bores opened up in a blacktop engine.
with shells and things, they were cheaper than over here, but i did get stung for import tax.
i'd plan on a grand for pistons and shells, £100 for boring maybe.
that will take over 500bhp without problems, oil pump will be the next weak link though.