Look for a Rolleimeter, a wonderful accessory that actually turns a Rollei TLR into a split-image optival rangefinder. Not a screen -- a real rangefinder!
Well... sort of! Be aware that the famous Rolleimeter will not fit all Rolleiflexes. It was made for Rolleis WITHOUT a removable hood (those made before the E and F models, as well as Wides and Teles). They show up on eBay, but you will pay dearly for this little GAS-item. I would not rate it as particularly practical (though Rollei used to list all of their accessories in a "Practical Accessories" pamphlet). It is also fragile and, trust me, Rollei no longer makes replacement prisms for it!
I think the Rolleiflex functions very much like a RF camera in certain situations. For example, I am fond of the open-frame sportsfinder aspect of the hood, especially when used in conjunction with the (built-in) diopter-plus-drop-down-mirror allowing focusing on the center of the screen... with an upside-down image. It's difficult to get lost in that kind of image, but it gives you a very quick focus determination. Oh, yes, that would be a two-window approach from the back of the hood, like a Barnack camera or FSU RF.
Another way that the Rolleiflex works like a RF is the ease of use of distance scale focusing and depth of field calculation. Preset the focus, and it's child's play to 'shoot from the hip' without even opening the viewing hood (OK, it takes some practice, but you can learn to do it).
It is great for street shooting, regardless of what you consider it to be. It is easily hand-held at relatively low speeds. (Bracing a Yashica-Mat on my knee, I once made a 4-second time exposure... though I wouldn't recommend that as a 'best practice' if you have any sense of image quality.) And Rollei shutters are quiet - really, really quiet. A gentle 'ping'.
And the image quality is superb - Xenotars and Planars are fantastic lenses, even used wide open. Tessars are essentially equal, when stopped down to f8 or better. Rolleicords with 3-element Triotars are noticeably less wonderful.
Careful - GAS builds up quickly in this area...
