From my research I would not get a Cayenne older than an '05 and then it would have to have some kind of dealer certified warranty (best) or an aftermarket warranty.
Not much traffic here, Rennlist has an active Cayenne board w/some knowledeable folks there.
I see you have a Disco II. I have a 1999 Disco II, which has just come off it's exteneded warranty. I have had it since new and have not really had many problems, certainly not anything near the reputation of them. In the last year I have had more issues, like both air bags on the rear suspension (it has ACE and SLS) needing to be replaced and I keep after the leaks. Transmission seals, head gasket, etc. I am glad I had the ext. warranty. I'll probably be moving it on in the next year or after something major goes wrong
Since you are considering an RR, I guess your expereince has not been bad. Wife kind of liked the RR Sport when it first came out. We were seriously considering the Cayenne, but after reading about some of the problems and Porsche's attitude along with our own bad experinces with the local Porsche dealerships, we have pretty much decided not to go that route. Seriously considering a Cadillac SRX. In the past I have needed the off road capabilites, but that has now come to an end, so that is no longer a major factor.
Good luck with your decision.