To turbo a 1.2 you'd have to custom build most of the parts I'd imagine. The best people to ask about that would be the cinque/sei ppl as they have done 1.2t before I believe. You'd certainly need to modify the exhaust manifold, air intake as well I'd imagine...and probably have either stand-alone management or a bloody good remap. I'd guess you'd also probably need a metal HG...oh and the engine doesn't want to be too thrashed either IMO.
GT Conversion costs more to insure than a GT usually because of the fact that parts have been swapped between the cars.
The conversion cost itself depends entirely on what level of cinversion you are looking for...and who is doing the work. Bear in mind that it's not just a case of swapping the block over...it'll need driveshafts, gearbox, ecu, rear beam, brakes and a few other bits. If you want to go the whole hog and get all the electrics swapped as well it's obviously cost more. You basically end up using most of a GT for the conversion...so buying a GT to drive rather than convert would always be cheaper IMO.