The Contax 645 is a great camera. It could easily replace both your 35mm and 6x7. I bought mine when they first came out in 1999 and have used it fairly steadily since. It recently needed a shutter adjustment, but that's the only problem I ever had with it.The f2 speed of the 80mm lens opens up all kinds of new opportunities you wouldn't think of for medium format, particularly with Ilford Delta 3200 or an 800 speed color film. The lens yields a nice image at f2, though center sharpness is best (80+ LPM) at f 5.6 and overall center to edge sharpness is best at f11.

Overall, it's designed well and handles more like a 35mm than medium format. Autofocus is slow by 35mm standards, but accurate. Find a good source of 2CR5 batteries since the autofocus really runs through them. I set the focus to manual and use the back button (thumb) to focus before each shot. This more than doubles the life of the battery.

I would suggest you use it exclusively for a while and then decide where your other cameras fit into the mix. I certainly wouldn't carry all three formats at the same time.

Danny Burk has a good review of the camera on his website. I wish you the best of luck with it.

edit: to actually answer your questions, the meter is very accurate both is spot and full frame mode. It also has a built in flash meter to make strobe work pretty fool proof. I bought a Lowwpro MF bag for mine it holds the camera, 3 lenses, flash and spare backs just fine.