For a good 250 engine you want idealy forged piston and rods, running it on EFI as your can tune it alot better. You will also need headwork and a cam, and as for the turbo you will need a decent stage T3 or a cossie one (im sure a few of the other lads can spec a better turbo).

Cossie managment basicly is your EFI engine running Cosworth Managment...ECU, loom, sensors, Throttle boddy etc...

As to what bottom end you want depends on what power you want...
You can get 250 out of a 1600 but that would be pushing it. After that you could go ZVH which basicly mean you run the standard CVH 8v head with a 2.0 zetec bottom end. If you wanted 300+ then you would need to build a zetec turbo.