Let's start with focal length. Although you will only be doing 6X6 negative at first, you may want to consider a lens that will cover the full 6X7 frame. In that case, you would want a lens in the 90 to 105mm range. The longer focal length limits the maximum size of the print you can make, however, so you need to look at how high you can raise the enlarger head. For an 8X enlargement to a 20 inch wide image, you will need about 33 inches for a 105mm lens, 28 inches for a 90mm, and 25 inches for an 80mm (normal for 6X6). In general, get the longest focal length you can live with.
As for brands, both Rodenstock (Rodagon) and Schneider (Companon) make excellent enlarging lenses. I suspect, judging from their other lenses, that the Fujinon enlarging lenses would also be excellent. Nikon (EL-Nikor) is also very good, and some of their older (but not too old, say 70s and 80s) products were outstanding. Bought new, a good enlarging lens can be expensive. But many are now available on the used market at very low prices. With the market the way it is, stay with the top quality brands mentioned above.
