I might suggest you consider a Mamiya 645 Super if you are interested in the 645 format (which it sounds like you might not be). It has interchangable backs, has a great winder available which improves hand held performance immensely and is quite a bit newer than a 645J. I have had two older 645s, an original 645 and a 1000s both go out on me, so I am a bit leery there. Of course I got them on eBay with no warranty.

Prices for the super are not much higher than the older models. KEH has a kit with unmetered prism, winder, back, and 80mm lens for about $350. Not sure what they would charge to ship to the UK, but it might be worth looking into.