the S12 actually had a CA18ET, not DET. it was the 89 and early model 1990 JDM 240's (180sx) and silvia's that had the CA18DET.
it either requires adapter plates, or simply a transmission from a kn13 pulsar SE, as it was the vehicle with the N/A CA18DE. it's my understanding that the DE and DET are quite different internally, which is normal with n/a and fi versions of the same engine. one is built for boost (lower compression, forged internals) while the other is built for n/a performance/daily driving. the b13 SR20DE and the s13/bluebird SR20DET's are like that as well. i'm sure the CA18DE can be boosted as well, but not the same level as the DET without heavy internal mods. you probably couldn't go over 6-7 psi.
and as far as using a GA-series platform, mine is the pain in the ass method. it could be better, but it will take longer and could be more painful.
if you want something simple, swap in an OBD1 GA16DE from a b13, and get the turbo kit for it upgraded to a T25, then use the b13 SR20DE ECU and harness along with the SR20DE 279cc injectors, and you have a system full capable of 12psi that bolts in without too much hassle.