A few points I would like to clarify/add to.
To further confuse matters, I've also seen a small version that uses 127 film (rare!)
The 127 film version was the Rolleiflex 4x4, but most of them are obviously smaller (as you would expect), and aside from early models, all of them have a very different paint job to the 'conventional' Rolleiflex/Rolleicord models. They look like fun, but command a small premium for some reason, and a similar priced Rolleicord IV/V/Va/Vb which takes 120 film would be a much better camera to own and use.
Just about all Rolleis ever made have the standard 80mm lens (some might be 75mm) but they did (do?) make a Tele Rollei and a Wide Rollei - but they cost serious amounts of money!
Franke & Heidecke probably produced more models with 75mm lenses than 80mm lenses. The earliest models, the Tessar-equipped Automats, all the Rolleicords, Rolleiflex T, all the 3.5 lettered models - all were 75mm. Even the unloved Rollei Magic was 75mm.
2.8 models are 80mm, 3.5 models are 75mm. They're great cameras and you can't really go wrong with any model if they're in good shape.
Definitely the case for
later models (think 2.8/3.5C or D onwards) but the Rolleiflex T is renowned for having inferior internal mechanical parts (Harry Fleenor is believed to be quite disparaging about this), the Rollei Magics are almost entirely reliant on often dead selenium cells, there was a large batch of 2.8As with mismatched lenses etc. etc.
On a budget of £100, I would look for a Rolleicord IV/V or a Rolleiflex Automat. The former are likely to have had less film through them though, and are younger, and offer a lot of Rollei for very little outlay.
