welcome to the forum
!!
regarding the mods on the m20b25 it depends on how much your budget is, and how many hp your goal is..
For example, for "around" hp you dont have to do anything with the engine internally, of course its better to do it if you have the budget for it, but if you dont you can wait with the internal mods until you have the money for it and at the first stage just buy the turboparts (turbo, turbomanifold etc)..
heres a topic you can read, its about turboconverting
http://www.pure-pf.com/phpBB2/viewtopic.php?t=1003 , lots written about the m20b25..
you can also find a "stage 1" kit there as well..
Stage 2 would be with mods to the engine internally, mainly lowering the compression ratio.. theres many ways of doing that, you can get a thicker headgasket, take of some material in the cumbustion chhamber in hte head, take of some material on the original pistons, getting other pistons etc..
then you should get other headbolts, complete renovation of the engine etc..
to much to write down
.. start of with reading the link I gave you.. it´ll help alot.. if theres something your not sure about, just ask
..