Stuart,
Keep your eyes on the classifieds on this site (and others). I wouldn't worry about rim, tyre size or offset as you're buying second hand and changing tow vehicles, if needed it can be done later. Make sure whatever it is you look at is sturdy, has eye to eye spring mounts (any off road work tests slipper mounts to the max) and the canvas is in good order (this can cost a motza). Zips can be a little problematic but no matter what make sure it has brakes and that you have the proper controller, You will always travel with it nearer max weight rather than min weight. If you are handy then the mods begin, if not spend a little more in the first place. Other than that if you can find a good Tambo Cooper then go for it.
Best of luck
Mark