there is an article on my web site about tuning 3SG's.
many of the power claims made by folk for simple changes, cannot be backed up with dyno tests. i have my own dyno so when i quote a value, it's based on fact
if you have to pay someone to do the work, then your choices will be goverened by your budget
any comprehensive rebuild of a 200000km old engine, with upgraded forged pistons, better cams etc will be $4k + for engine alone. The price is much the same for NA or turbo engines, with NA slightly dearer due to special pistons needed
turbo swaps are only sensible if you have a half-cut. even then expect to pay someone around $2500 to do the job + half cut price
Fitting a later BEAMS 3SG is a great conversion , but again expect to pay around $3000 to do the job + half cut price
A drive-in drive out 2GR V6 converion is the ultimate