Here’s a writeup I did on this in these forums last year. If you’re just digitizing 35mm, the 40mm Nikkor with ES-1 will work well. RawTherapee is excellent, and use your base ISO (100 for the D7200?).
Clean your slides well! You can use a good bulb blower for loose dust, but to remove those hundreds of tiny specs of embedded dirt, use 3M Magic Tape. That will take 15-20 seconds per slide and will save you a lot of headache later on.