I use a 90mm Beseler Color Pro that works well - just be sure that if you are considering the Beseler lenses, it is a Color Pro version that you consider.
If you are happy with a 50mm El-Nikkor for 35mm, you may want to try for a 90mm to 105mm El-Nikor of similar vintage as well, because it can be useful to have similar contrast and ergonomics across your lenses.
I would probably avoid the 80mm El-Nikkor for 6x7, even if it has the coverage. You may find that the additional enlargement from that lens will mean uncomfortably close working distances for small prints.
For some reason, it seems to be slightly more difficult to find 90mm lenses as compared to 80mm or 100mm or 105mm lenses. Unless you are planning to do a lot of large (16x20 or larger) enlargements, it probably doesn't hurt to settle on a 100mm or 105mm lens instead of the 90mm.
You can go even longer (135mm for instance), but that will make it difficult to do larger prints, and some enlargers will need special lensboards to use the longer lenses, because they don't have enough bellows draw.
Their are other lens brands worth considering (Minolta and Meopta come to mind), but before buying anything that hasn't been specifically referred to here, you should make an enquiry here. Some of the manufacturers have several different quality lines, often with similar sounding names (e.g. Componar, Componon, Componon-S), so it is a good idea to check first.
Hope this helps - and have fun!
Matt