Qucik questions on Differential, tranny, Solenoid C change options

[redsvx94]

My front differential is shot, and I’m contemplating between installing used F. Differential or used forester/legacy/impreza transmission and rear differential. It would be depending on which part I could find very soon in reasonable shipping, but I have the following questions.

If I want it to be a drop in, and if installing whole transmission and rear differential, what model year, the donor car should be?

Once the mechanic drops the transmission, how much more $ should I pay for replacing duty solenoid C? ( if it is just run-of-the mill tranny shop not suby dealer) Could any one point me to a step by step direction on sol C change, if available? The shop says ~2 hrs and not sure whether this is a reasonable estimate.

Thanks!