Answering the second question first, and keeping in mind that both "good" and "won't break the bank" are subjective, asking a lens to be good from wide to tele is asking a lot. (And sort of defeats the purpose of an interchangeable lens system, IMO.) Ask for that lens to also be cheap, and now you cross into the realm of asking too much, IMO.
That said, have you looked at the Olympus 12-50mm lens? IMO, it's slow and overpriced (and obviously barely telephoto with the 2X crop), but it's the closest lens I can think of to your criteria. (e.g. both the Panasonic 14-140mm and Olympus 12-60mm cost more; in the latter case, significantly more, and that's not including the adapter.)
And that brings me to: as noted above, you can use any micro 4/3 lenses regardless of brand (I personally mix and match lenses from Panasonic, Olympus and Voigtlander on my Panasonic GH2), and you can use (standard) 4/3 lenses with the adapter Carlos linked to. There are also "dumb" adapters--called that because they lack electronics--for just about any lens mount you can think of (I personally use one to mount Canon EF lenses on my GH2) but since they lack electronics, any lenses mounted this way will be full manual only. i.e. no AF, no electronic aperture control, no IS, etc.
HTH - Mark