It's pretty much been shown that on a 7" wheel with the appropriate tire you can safely run as low as 45mm. If you're not lowered I'd say you can go even more, perhaps 42mm. Some have run 40mm offset but personally I think it's way to boderline to bother if buying new wheels.

As for 55mm offset wheels I'm sure you can find a few sets of japanese wheels that will have this offset.

Any particular setup your thinking about for wheels/tires?