Rolleiflex viewing lenses, as Frank said, are almost always f/2.8 - exceptions are the old series, Automats, equipped w tessars or xenars. But the brightness difference is not too big.
What also should be added, is that a f/2.8 rolleiflex weighs much more than the f/3.5 brothers. I mean, the difference can be as big as 1.6 kg versus 0.9 kg. Automats are even lighter.
Automats are a very good value for the money. The tessar/xenar lenses are excellent. The most important thing that comes when buying an old/ish TLR is to have the lens properly aligned. Should be parallel with the film plane AND should be calibrated to the viewing lens. If this is not perfect, the Planars and Xenotars can produce very bad photos.
"Automat" refers to automatic film frame counter, all the rest is fully manual on the camera.
Rolleicords are cheaper but still very good. Well built, great lenses; only they are a bit slower in operation and always have the dim screen (unless sbody switched it).
Mamiya TLR's are great for the lens system, but you need a cart and an ox to carry the whole system with you. If you can live with that, they are very very cheap for what they can do.
