Keep it simple! I have made thousands of internegatives, both B&W and colour over the course of a 20-year career as a photographer and lab technician.(posted 9151 days ago)I would make internegs on 4x5 sheet film for the best quality. The best film for this purpose is no longer made, namely Kodak Super-XX Pan Film. I have made good inegs on T-Max 100 film, although they can also be made on such medium-speed films as Plus-X Professional, Ektapan and FP4 PLus.
You will have to derive specific development times based on your enlarger and enlarging lens. Generally speaking, a diffusion enlarger will give you best results when making the internegative; avoid using a condenser enlarger, mostly due to dust problems. If you opt to make the inegs by contact, then you will have the sharpest images as well as LOTS of problems with dust! Any dust on the ineg will show up as black marks on the final print, which will require the ineg itself to spotted/retouched.
Bear in mind that no matter how carefully you expose and develop your inegs, you will loose a bit of separation in the shadows and highlights. What you are doing is compressing a density range of something like 3.0 or greater from the original transparancy down to about 1.2 or less and then expanding it out again when you print it. It is sort of like taking freshly squeezed orange juice, dehydrating it, and then reconstituting it with water! The shoulder and toe of the transparancy have less contrast than the midtones do, and compressing them and then expanding them out will cause a bit of quality loss in the final B&W print.
I wouldn't use a colour interneg film and make prints from that; while I have never gone that route, the potential for problems is much greater than with B&W film.
I hope this helps; if you have any specific questions feel free to e-mail me.