I have a B8, which apparently lacks the "auto-focus" of the B7, but is otherwise very similar and I'm quite pleased with it. I use EL Nikkor enlarging lenses -- 50mm, 80mm and 105mm -- for 35mm, 6x6, and 6x9. For 6x9, the corners are a bit iffy, but at 6x7 it should be great. I have had some intermittent behavior in the cord to the lamp socket, but that is seldom a problem. It normally only shows when doing a major height change, and rearranging the cord brings it back to life. It could probably be fixed if it bothered me often enough. It uses a PH111 lamp, 75 watt, a bayonet base like a frosted auto tail light bulb in shape, which so far is still available.
There is a broad assortment of negative holders, just make sure you get the one(s) you actually need, if possible. There are two "supplementary condensers" (assuming the B7 is like the B8), a thin one for 6x6cm/80mm, a thicker one for 35mm/50mm. Without major testing, it appears you could live without those, perhaps just lose a little light with the smaller formats. The 75 watt provides enough light that I use a 2-stop ND filter for small (5x5, 5x7) prints, so a bit of a light loss is not likely a problem. Also the 50mm lens for 35mm negatives works with a flat mounting plate. For 80mm and 105mm, you need a sort of "nose cone" -- upside down top hat shape to space the lens out for appropriate focus. Those ancillary items seem to be somewhat rare (and/or pricey) but I was able to make serviceable substitutes. Ideally, you will get some of such items with the package. (I have no idea how or with what they were originally set up for purchase.)
They are pretty old beasties, but the mechanics (at least, non auto-focus) are pretty simple, so as long as nothing is badly corroded from unhappy storage, I think you should be in good shape. The baseboard on mine had a lot of old tape and marks on it, so I removed the column, the deteriorated rubber pads on the bottom, and the mounting plate, then refinished it with wipe-on polyurethane. It looks like a new one.
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