Hi
I have an Sqam and Sqai but cannot comment on the others you mention.
I love both and have, in 6 or so years of moderate use, had no problems with either. Many say the Am is too reliant on the motor drive and batteries etc. but it's simple electrics and they do tend to be cheaper than other models, handles more like an SLR as it has built in grip - heavy with prism finder but if you want light best go with 6x4.5 .
The Sqai is the top of the range but really only gives one extra feature which is off the film metering for flash - just flash - with the special adapter and metz flash!
You still need a handheld meter or metered prism for any other situations. This is also at the expense of a rather temperamental battery compartment that needs several small bits of card jammed in to function most of the time. I if was buying again I would go for the Sqa (not i) which does not have this.
The macro lens is rare and expensive , there are two versions 1:1 and 1:4 so look carefully if one seems too good to be true.
Extension tubes or close up lenses could be a simpler option.
When buying backs be aware that this is the main point of problems , the interlink between the back and camera, not too hard to fix if your that way inclined but best just pay a bit more and get one that looks in really good condition. I have personaly not had any trouble with the 5 backs I own.
Hope this helps.