you need low compression, decent conrod bolts, exhaust manifold that will take the turbo, better valve springs, good inlet that will flow well and take high capacity injectors(ie Rover T series inlet/820turbo inlet), management that can 'see' the boost, block drilled for relocated breather and an oil return for the turbo, intercooler and plumbing, k+n filter or similar alternative, focus rs 'cold' plugs

hope this helps as a rough guide for a budget build