I currently have a Mamiya 645 1000S and like it. I does have it's drawbacks like being heavy (really heavy after carrying it all day), no built in metering, obsolete -as in not being made anymore so the only accessories I can find are used. Which leads me to my first question. There are a lot of good deals on newer models in the used market. Is it worthwhile for me to consider upgrading to one of these?

Currently I don't have that many accessories that would hold me back. I have the power winder, several 120 film inserts and several lenses. I don't think the power winder is going to move forward to a newer model, but I bought that more as a convenience thing to make it easier to hold, focus and shoot. The film inserts aren't that expensive, but I would like to be able to keep the if possible. The big question in my mind is whether the lenses will be usable on a newer model.

As for other questions here goes:

What model camera would be good to consider?
At what point in the model progression would the lenses no longer be usable?
Would I be better off scrpping this whole idea nad going to a different brand?