The main competitor for the Rolleiflex is probably a Hasselblad. The advantage of the Hasselblads is they are entirely mechanical, and thus repairable. The Rolleis are electronic, and some earlier models are said (on the internet, for what it's worth) to develop electronic failures. The downside of the Hasselblads is that they are either much more expensive, or very old. Getting them repaired is expensive.
I would make sure to get a Rollei that has the latest generation of their electronics: 6008 integral, 6008E or 6003 professional. They were all built from the mid-90s to early 2000s. The latest model is the 6008 integral 2, which was built until 2015 or so. The difference between the 6003 professional and 6008 i2 both of which I own is small. The display is updated, and somewhat nicer in the 6008 i2. The 6003 does not have an exchangeable film back. The advantage of this is that the camera is slightly smaller. Changing the film on the 6008 i2 requires you to close the built-in dark slide. This is an additional part the 6003 does not have, and thus a potential point of failure. Film backs are hard to find and expensive. In terms of use, I am pretty indifferent between the 6003 prof and 6008 i2. I prefer the film changing mechanism of the 6003, and the display of the 6008 i2. You can attach a grip to both of them for easier handling, but to be honest, I prefer shooting without the grip.
The lenses are pretty much comparable to he latest Hasselblad lenses, sharp and contrasty. If you prefer a more vintage look, there are probably better camera systems out there.
Whichever version you get, make sure it comes with a battery.
