20's take a serious beating...
td06+20 was one of my favorite setups. If you go with a td06 set up on the 20, then you can use the same manifolds, plumbing etc when/if you go to a 25- then upgrade the td06 to a t67 and everything will bolt up to it.
anywhere + of 220rwkw with a 20 is where you start spending money... nistune, z32, fuel pump, injectors, coil packs, fmic, oil cooler, decent exhaust, and a turbo set up. All this gear will line up on a 25 if you upgrade too.. Don't bother changing the standard inlet manifold on a 20 either.
Justin.