Another good option for low light is Mamiya C3 series TLR. Same idea as Rolleiflex 2.8, but a lot less expensive.

I've shot a lot of wedding on Mamiya 6. I have two of them, and I always had them with different lenses on. Many people mentioned mirror vibration, but for church ceremony, indoor reception, etc., the problem is more to do with the subject's motion blur. You're more likely forced to use flash with medium format than with 35mm, where you have f/1.4 lenses available.