I have a Rolleiflex 3.5E2 Planar with non-interchangeable screen -- but the camera was CLAed by Harry Fleenor and the screen replaced with a Maxwell at the time.
Things to look out for:
If you get an E2 or earlier model, you'll need to have the screen professionally replaced (the original vintage screens are pretty dark, IMO). E3 and F models have user-replaceable screens.
If you buy from someone who has not been using it regularly, assume it will need a CLA to be usable: shutters on these cameras grow sticky and inaccurate with disuse, and the automatic frame-stop/counter feature of the film transport often malfunctions if not clean. Also, if it is not in regular use, or you're buying from an unreliable source (e.g., eBay), check carefully for lens element separation (particularly a problem with Xenotar lenses), haze and fungus. Also check that the lens stage is not out of parallel with the camera body (due to being dropped or manhandled by someone not familiar with the camera).
When buying accessories, make sure you get the right Bayonet version for your camera. For example, the 3.5E and 3.5F use Bay II but the 2.8E/F use Bay III. (And for the hinged double lens caps, the 3.5E and 3.5F have slightly different spacing between the lenses, so their caps, while nominally both Bay II, are not interchangeable.)
Definitely buy a Rolleifix accessory -- kind of a dedicated quick-release adapter, which is very helpful even if you use a tripod with its own quick-release plate.
If you can't find a vintage Rollie strap with the alligator clips (or don't want to pay Luigi to make one for you), there's an OpTech neoprene strap that has simple loops at the ends that can easily be attached to the strap lugs on the camera.
The LowePro Nova Mini AW is an inexpensive bag that's a perfect fit for a Rolleiflex with a few accessories and some film.
Enjoy!
::Ari