4g63 = 2.0 sohc8v/dohc16v/dohc16vturbo I4
4g64 = 2.4 sohc8v/dohc16v I4 (can accept 4g63t parts to make a 2.4t)
the above two are from the sirius mitsu engine family iirc
4g54 = 2.6 sohc carb and turbo I4

I would build the 2.4 over the other two, and perhaps over a 3.0 12v, even if I was turboing the 3.0. The 2.4 would be a lot easier to turbo, as no exh manifolds have to be fabbed up, most all the parts are stock mitsu except the ecu (and you can even use that if you fool the maf by 20%).

All that's left is water plumbing, fuel plumbing, downpipe back exhaust, and a control cables (throttle and if a/t, tv cable) and wiring it up. You have to do all that for a 3.0t PLUS fab up some heavy iron exh mani's (or at least one if you don't TT) or be prepared to weld up cracks on tubular manis every month from now on. The 2.4 makes impressive power, and there's a huge aftermarket that supports the 2.0 and 2.4 engines, especially in turbo form - large forum communities, too.