As I have done some things myself, I might be able to shed some light on your plans:
1. Everything depends from the tire size, lowering, J, ET, wheel design and other stuff. In my opinion 16" is the best size, I have tried 17"s and they did not look that good.
2. About the federbeine - well, as far as I know - everything with 1-piece cast iron parts will do as long as it is bigblock engine (1.6 and bigger). Personally I did it with Kadett E 1.6S (16SH) suspension legs, I believe it should be possible with Ascona C, Astra F, Kadett E, even Vectra A and Calibra struts as long as you keep the D Kadett upper support bearing (domlager, I believe, its called) and also keep in mind the possible differences in shock absorber rod diameters and lengths. I used, as mentioned, Kadett E struts, later replaced the shock absorbers with Astra F GSI ones. My mate installed Vectra A shock absorbers in Ascona C struts, had to use some spacers below the shocks due to Vectra shocks being shorter and also had to drill upper support bearing from 12mm to 14mm (that's, of course, if I recall everything correctly ). If you have all the parts, you will see what is necessarry to make it work.
4. On Kadett D only 42L tank will fit without any modifications to the car shell itself. That being said, look for Kadett E GSI 18E tank (4/5 door version, prefacelift so to say). Or for starters you could modify the sender unit on the existing tank by soldering the copper tube in it. It's shenanigans but it works for some time. I have started to convert my next Kadett shell for 52L tank with the internal pump, you could find my pictures in my Kadett D thread.
If you have more questions, I will be happy to help if I could.