Have seen some 20V turbo cars running around since sometimes ago, I'd say keep it at low boost, says around 6-8psi, if you are only gonna use the stock 20V ECU. The ignition timing is always the problem and you won't want to blow your engine when you are having some fun at the lights.
Turbo size depends on how deep is your pocket but for a normal setup, a TD05 range of turbo should do the job if you are looking for low end torque and early 4AGZE pistons set is the one to go due to the low compression, otherwise fly cut your stock pistons, but that's not always the way it should be...
Intercooler, fuel injectors, higher flow fuel pump, AIC are the things you might want to consider. Over here people are using GZE block with 20V head, more reliable, due to sometimes people just seem to be unable to control their foot on the gas pedal properly, especially when challenged by the car next to you...hahaa.
However, since there are lots of monster cars rolling in town now, I doubt you will satisfy with a simple low boost bolt on setup. Might as well keep the money in your tabung and go for a proper turbo setup later on.
Torque is very addicted, and it's not impossible that you will greed for more.
1978 TOYOTA CRESSIDA
2005 PROTON GEN2