I think it's really the only option you've got. I'm not really into the Olympus/Panasonic/Four-Thids-system (I'm a Canon-user), but a little research showed that lens is the only zoomlens in that range in the selected price range.
As said, I have no experience with this system, but I think this lens is a fine one for the more budget-minded photographer. If you're looking for real quality (comparable to Canon's L-range) though, you've got to save some more money and try to get your hands on a 50-200 F2.8-3.5.
Considering your Sigma-question: they do make lenses that fit your camera.
Here's a nice overview. Maybe you can also try to get the Sigma 70-200 F2.8, which really is a great lens.
Oh, and I think it's a hard job to find Olympus-users here. As far as I know, most is Canon, Nikon and Sony.