Wait are you thinking about ditching the ATW for the front mount? or using them at the same time. The top mount ATW with a properly sized turbo will be more effective and have less boost pressure drop which will allow for more power, and faster response. The front mount looks big, do you know the core thickness and inlet outlet sizes?
I am on the tail end of saving my ST185 and I have been tossing the idea of front mount VS TOP. For now I am using a CT26 with a billet Garrett GTX compressor wheel and will use the top mount with a fan. If it does not cool effectivly I will be considering ATW but it will be a custom unit built from scratch.
As for motors you say rebuilt VS a used. I been doing high performance cars for a long time and the term rebuilt is loosely used and often farse. Unless you have the paperwork, blue printing or receipts it's hard to say what you have. If you have those I would use that motor and sort the issues you have. I picked up a R33 with a fully built and blue printed motor last year and it was a wiring mess and didn't run. A new harness, time, and schematics I turned a 20K trade into a 40K car. Just keep that in mind.