Brilliant bikes, very underrated. Unswervingly reliable. Robust enough and quite capable off-road. Great handling on-road.
Basic run down of models:
RD03 - 650cc 1988-90 (?) and rare in the UK
RD04 - 750cc 1989-92 (looks very similar to an RD03 but has twin headlamps, twin front discs, etc.)
RD07 - 750cc 1993-1995 (a very different looking bike from the preceeding ones)
RD07a - 1996-2003 (minor changes to bodywork, ignition and carbs)
I have a '94 RD07 on which I've done nearly 20k on and off-road miles (inc. Stella Alpina) in the two years I've had it (it's now showing 55k miles), and nothing has gone wrong with it in that time. The only time I've was stranded was when a rear wheel bearing collapsed about a month after I bought it.
Definately check out
www.xrv.org.uk - lots of useful knowledge on there.