proper bottem end - ie steel pistons and rods

t3 turbo (although with your worry about lag this may not be a wise choise unless you have it built with roller bearings)

and better managment - maybe cosworth L6 or L8 or aftermarket if you can afford it

but this wont make the engine come on boost much earlier - maybe a lighter flywheel as this would help an engine rev faster so you could get on boost quicker, other than that you could get some heat protection on the turbo/manifold to help it come on boost quicker as well

how much have you got to spend?

Jonny S2 - i was reading something on turbos the other day in redline magazine and they said compression doesnt make much of a difference to lag and you could do lots of things before lifting the compression to get the turbo to come on boost faster