In answer to the original question, they are all 120 in 6x6cm format. The Rolleiflexes always command a high price, especially the late model F's, with the 2.8 version going for the highest amount.
All Rolleis are beautifully built, so buying an older model shouldn't be a problem if it has been looked after. Also, don't discount the Rolleicords as they are also fine picture takers, just have knob winds and the viewfinders are somewhat darker.
The Rolleiflex T is usually a good buy also, but you will find prices on ebay usually very inflated, so keep an eye on the classifieds here as Apug people are more in touch with reality (mostly) and are more knowledgeable about condition etc.