With regards to multiple exposure on a 500CM (or any other Hasselblad V camera) the process is:
1. take photo
2. put dark slide in
3. take back off
4. wind camera to cock shutter and return mirror
5. put back on
6. take dark slide off
7. repeat for as many exposures you want
I don't consider this an issue as I've made maybe 3 multi-exposures ever and the first one was just to try it out. Obviously if you shoot a handful of such shots every week then you should seriously consider a proper multi-exposure camera. It is not just inconvenient to do such action on the 500CM but in the end you put the slide in/out and the back on/off all the time, wearing down the light traps and the back latch and you're also much more likely to drop something if you're constantly fiddling it.
As for which Bronica to buy, I'd say the newer the better (as with everything). Here's the timeline for Bronicas
http://tamron-usa.com/bronica/sq_guide.asp
You can also consider the SQB, which is the newest, if you're not going to want a metered prism. I don't know what the prices are like there but a SQ-B in the UK is almost half the money of an SQ-Ai, yet the SQB is newer. Also, a SQ-Ai is nearly the same money as a good condition 500CM.
Regarding what the 500CM (and any other 'blad) cannot do vs the SQAi:
The 500CM won't couple with metered prism (you can use one but there is no automation)
The 500CM has no motorised winder
The 500CM doesn't do multi-exposures
The 500CM doesn't have 135 backs (if you're into that thing)
If you want a grip, the 500CM has a
left-hand grip for the shutter and you then wind with your right hand using the body crank. The SQ has a right hand grip with a crank to wind the film making holding and handling as a large 35mm SLR.
On the plus side the 500CM will:
Take Zeiss lenses which may or may not be of tangible difference to
you
Work without batteries, which is either an advantage (it just works!) or a disadvantage (the lenses will eventually need servicing after a number of years)
Reliability? Don't know. I've had two old Bronica lenses go bad on me. I've had an even older Hasselblad back needing repair. Is that a draw? Bad luck? One vs the other? I'd say they both look and feel like they'll last many years so I wouldn't consider this an issue. These are both old cameras, decades old, so expect some servicing to be needed in the long term.
I also find the Hasselblad design much nicer to hold and see and use, but that is obviously of no significance whatsoever towards the final image.
Finally, you seriously need to consider your budget. You can get a SQAi or 500CM kit for about the same money. But adding anything after that is
VERY EXPENSIVE for the 500CM. I can get SQ backs for £40. A usable 500CM back is at least double that or £150 if you want to be sure it won't need new light traps. Lenses for a 500CM are about £50-100 more expensive when you compare the newest Bronica PS lenses to the oldest Hasselblad C lenses:
A Bronica 150PS is about £150-200.
A 30-40 year old Hasselblad 150C is £200-250.
A slightly newer 150CT* is £250-300.
A more modern 150CF is £400-500.
The current version 150CFi is £900 and more.
I can get a SQB and waist finder, 80PSB, a couple of 120 backs and a 150PS for the price of a 500CM. That's some serious money that can go towards film and processing.