I had one and traded it away a few years ago. I now wish that I hadn't done that. These are wonderful cameras. The usual complaints about Retina are the fiddly character of the film advance and the fact that the front element of the lens is what changes if you want to use on a different focal length. The cost and trouble of finding someone to work on them is significant, so I wouldn't get one that is nice looking but has problems. The lenses for the Retina Reflex (just the Reflex, not the later versions with numbers after the name) will fit, but you can't close the camera with anything but the original 50 MM. The f2 lens is very sharp, and I haven't really seen much difference, other than that extra half-stop of speed, in the f2.8.
Sometimes you find Retinas in antique shops that are supposedly broken, when all they need is to have the film counter reset, but that doesn't happen as much anymore. The cost and trouble of finding someone to work on them is significant, so I wouldn't get one that is nice looking but has problems. They were very expensive cameras in their day, and sometimes have been well cared for and then put away as part of an estate, but the fact that the shutter is so intricately connected within the body makes them a nightmare to clean and lubricate unless you are very well versed in how they work.