I'm running a Rivage cab (mk1 shape) on a J plate, depends on which shape you like, VW never made a mk2 cab. Personally I prefer the old shape. If buying one make sure that the hood is nice and tight when up, i.e doesn't have any gaps between the window and the roof and has no rips. Check the carpet for any dampness, if there's any leaks water will find it.
Vixter left some advice on another thread about cabs the other day, a couple of good ones were, if the owner will let you, take it to a power wash and give it a once over, this will soon determine if there are any leaks, also if there are a load of air fresheners/smellies in the car they could be hiding the smell of damp.
They made mk1 cabs until 93 so they will be up to a K/L reg, they didn't do electric hoods until 89/90 (I think), its best to have that believe me. The best models of the mk1, in my opinion, are the Rivage (biased I know) or the sportline. The Rivage was done in dark blue or Green (with leather seats) and they were made from 91 - 93. I think, but not sure, that sportlines were only black or red, they have recaro seats and BBS alloys (I think again !). The sportlines are the sportier looking model but I don't know what year they were made from or until.
As for the newer cabs, sorry as I don't know anything about them at all. If you like the mk1 shape though there are usually a few sportlines and rivages on autotrader.co.uk. One summer in a cab and you will never want a hard top again ! Good luck in your search, hope you find what you are after.
One last thing, get it well alarmed, well crook locked and do as much as you can to secure the vehicle.