Built short block from Subaru from memory would cost €2-3k
Second hand, you'd probably get the long block for €1k or less, then you can sell the heads yourself. But while you have the short block, you'd be best upgrading the pistons and rods to forged ones.
UK Hawk wrx/sti have 2.5's as standard, some foresters also have them. Not sure on the difference in specs though