I love my Super 23! It takes all Mamiya 23 backs. The Universal had adapters for Mamiya or Graflock backs.
I have the 50mm lens with finder, standard 100mm lens and the 250mm f/8.0 lens. The viewfinder adjusts for the last two. I know there are unkind words about the 250mm f/8.0, but I get good results. It doesn't couple to the rangefinder, but it's generally used at infinity most of the time. I understand the 250mm f/5.6 is better, however I've never had the chance to try one. I do know they're a lot more expensive.
For a good overview of what was available and how it fits together, check out:
http://members.cox.net/gmhsint/mamiya.html
As Benno says, don't forget to pull the dark slide. (I have some great wedding photos on mine, I just can't figure out how to develop it!) Nothing is automatic or connected. Setting the right viewfinder mask, cocking the shutter, winding the film, pulling the slide, etc. are all seperate operations you have to do. Not unlike a view camera.
Other items to consider are extension tubes, which with the bellows back and groundglass back work good for close work. Masks for the viewfinder and film holder (certain ones only) allow 6X6 and 4.5X6 photos. A couple of different flash brackets. Tripod adapter that goes on in place of the hand grip to take vertical photos (which may allow interference between the dark slide and tripod head - mine does). Along with the ground glass back are film holders for 6X9 film. I use Efke. Handy when you do close-up work or quickly need just a couple of shots. Of course then you need a sheet film tank, etc.
The other advantage is, if you carry it in the field, you will build muscles, as they weigh a ton, but are built like a tank. Have fun!