Hasselblad lenses are "slow", just like all leaf-shutter lenses. You generally get f/2.8 on the standard lens and then it is f/4-5.6 on everything else. If you want faster lenses get a focal plane body. But...the lenses are "fast" enough for DOF control. You don't want faster than f/2.8 unless you can really focus very very finely. As for darker conditions, yes it is limiting just like anything else. Add the fact that an SLR (vs your TLR) will need faster speeds and you are "worse-off" compared to a 35mm SLR or a TLR or whatever. But you don't buy a Hasselblad/RB/RZ for hand-held night shots.
1/500? Well it can be limiting in bright daylight if you have 400 or faster film and you want wide open shooting. On the other hand most negative 400 films can be shot as 200 or 100 without issues.
I love my Hasselblad. It is expensive, pretty, fantastic to hold and the contrast and colour rendering is wonderful. Would I be happier with a RZ67? Don't know, I'd probably be happy with anything. I was happy with a Bronica SQ. It is just that the Hasselblad makes the whole experience nicer.
Another thing, I really don't know why people compare the RZ/RB vs the Hasselblad. You either want 6x7 or 6x6. If you want 6x7 get the Mamiya. If you want 6x6 get the Hassy/Bronica. Yes, if you have 6x7 you can crop to 6x6 and still have the original 6x7 to play with but then again I never wanted 6x7 to begin with.
Which 80? As with anything camera related, the most expensive one you can afford. Go for newest/best condition till you can't afford it. Otherwise you'll forever agonize whether you should get a "better" one. I have a 80CB. It is great. I also have a 50CT, 150CF and 250CT. They are all great. If one day I can afford to upgrade the 250 to a CFi I will. But I love the finish of the 50. Whatever that metal they made the old CTs from is is fantastic to hold. The 250CT is like that but the ergonomics are a bit lacking due to the size.
Basically, get a newish-body then get the lenses, send it all for a service and use them. They are mechnical cameras. They can be fixed and work like new.