Both the 670dxl and the 6700 use a reversible 39mm threaded lens board/mount ring to provide focusing for 35mm and 6x7 negatives.
Most enlarging lenses from 50mm-105mm's use the 39mm thread mount and are interchangeable. Older lenses or ones with a even shorter length usually have a smaller mount. Longer lenses used for large format have a larger mount.
Since your goal is to print 35mm, stick to the 50mm 2.8 lenses. El-nikkor, Rodagon, and Componon are versions made by Nikon, Rodenstock, and Schneider, and are ones you want to keep an eye out for.
Good luck on your enlarger search, you can try posting a WTB ad on the classifieds for one.