Just wanted to add some about Rolleiflex 600x:
Rollei also makes a revolving 6x4,5 back and the standard screen has guide lines for it. Since you can change between horizontal and vertical images by turning the back (while mounted on the camera), you can use the waist level finder even with 6x4,5.
Another thing that might be of interest is the M39/M40 shutter adapter that will let you mount enlarger lenses (only for close-up i think). You can also get a retro adapter so you can reverse mount the Rollei lenses. It will transfer the aperture from the lens to the camera or set the aperture if set to shutter priority.
For portraiture, both Zeiss 150/4 and Schneider 180/2.8 are good choices.
For architecture you have the 55mm PCS Schneider Super-Angulon lens with shift and tilt, but at $9000 (!) it will empty most peoples pockets.
You may hold it like you do with the TLR, or use a separate grip and/or a neck strap.
More information on
and of course http://www.rollei.de
The only real problem with this camera is the price. No wonder I'm still stuck with the one lens included.