The SR20 gearbox doesn't fit the CA18DET.
Its the same family of box which means the CA bellhousing is interchangeable. So ya need a spare bell housing for it to be fitted.
The SR20 would be very easy to fit
I would just get an SR and box together.
The S13 SR20 engine came with a T25G. The S14 SR20 engine came with a T28.
The heads aren't changeable without some level of work. It would be harder to fit a DET head to an ET as opposed to vice versa