You can't go wrong with using a T2 flange on the manifold. These days there are so many turbos that will bolt to a T2 flange your options are endless.

The other benefit is that, of the nissan/garrett range, the smallest turbos have a T2 flange. For applications like an A12, even a turbo like the T2 from the E15 would be sufficient. And then for the people who are looking for big power from their A series (gearbox permitting) the T28 ball bearing from the S15 would have no trouble keeping up.

Anybody who has a daily driver with a turbo on it will agree that there is no point having a big laggy turbo. You want good response which means choosing the smallest turbo that can keep up with your maximum flow demands.

I reckon for the sake of trying to keep the kit on the cheap side the CA18 T25 would be the best combination.

For you do be running an intercooler it would mean that you would have to be using a blow through setup am I right?