www.camcorderinfo.com has a good quick rundown on models per make. The Sonys and Panasonics appear to have the same or roughly the same model numbers as the UK ones (In Panasonics cases the American designation is NV-PS as opposed to NV-GS).
Big optical zooms on cheap cams are bad news, if you can afford it go for the xm2, the lens is flourite and so suffers from very little in the way of fringing (chromatic abberations where your subject has green and red edges) whereas any cheaper cams will have this.
Yes, but with teleconvertors you lose light, the xm2 can maintain f2.8 throughout it's zoom range, the second you add a teleconvertor you are looking at an effective f-no of 5.6, so you lose light, maybe adding gain if in low light (so further degradation), and you'll still get fringing.
Buy an XM2, go on, you know you want to.