remapping the ecu isnt bad if thats what your thinking, at most your looking at around 10bhp from the 2ltr unit.
if you do go down the turbo route, will come onto in a miniute, you will possible need a custom ECU rather than a remapped one anyway to take into account all the extra varibles.
have you looked a getting the cam profile changed to a slightly higher lift duration one, this may require a remap to get the best from.
ok onto the turbo - not heard of this done, but yes most turbo conversions are difficult to do and not something you could do yourself. basically you will need to custom make the intake to the turbom, pipe work from the turbo to intercooler then onto the inlet manifold. this will proberly require a bigger MAF (if fitted). most likely larger injectors will be required as well
custom build exhaust manifold to the turbo and from the turbo, ang wth a larger exhaust to get rid of the gases themselves.
the internals of the engine themselves may need strenghtining as well as maybe lower the compression to a more sutible amount ( look around 8.5 -9:1) there are different ways of doing this.
then the fun of mapping the whole thing together.
another option you may want to consider is supercharging, this does away with all the exhaust plumbing at least and is possible to connect up without changing much else other than binning the A/C compressor.
do a search for threads started by claymore one called the marex volumes i think.
he supercharged his engine using a eaton supercharger from a mini cooper s. it was still running on a standard maf and map with around 20bhp gain