If you want that excellent ride quality, then I think the P38A is the way to go. I was reminded of this recently when I test-drove a few that I hoped to talk my wife into buying. The P38A rides a fair bit better than my D2 V8 with air rear springs and 18in wheels...
Despite the ride and many other qualities, I would think twice about owning a P38A. I have tried hard to talk myself into a P38A - I really like them - a lot - but to be honest, they appear to be either a massive time-consumer and/or a money pit. Put simply, if you have a mortgage and kids and no time to tinker and are relying on the car to get to work, I would not recommend the P38A on what several people have told me. I have not actually owned one ( I have the kids/mortgage/no time problem), so take that info for what it's worth, but I am a P38A enthusiast. Do a seach here and you'll find heaps about the cost of P38A ownership and other info. I hear $5k a year is a good budget to expect for repairs. I reckon a Disco 2 should be under $2k a year for maintenance, by comparison.
I heard that when the P38A was developed in the early 90s, that it was owned by British Aerospace, who had no money. The shortcuts they took was borne out in the dodgy eletrical dramas the P38A can have. The Bosch electrical upgrade did no include the body control module etc so it only partially addresses the problems.
The biggest (most expensive) single problem that the Rover V8 of these later models (both D2 and P38) can have is slipped liners, which costs in the vicinity of $8k-$10k to fix. I am thinking of doing a poll to see how many here have had to replace/top hat their engines because of this problem. As far as I know, if you have a later engine with a cooling system kept in top shape (radiator and hoses replaced) you are unlikely to strike problems with this, but maybe others here can chime in with more precise info.
Have a look on rangerovers.net for heaps of RR info.
I don't want to put you off the P38A if that's the way you are determined to go - I may still own one, one day myself - but I would say a D2a V8 (2003-2004) would be better - more reliable, less maintenance hassles, more convenient interior for kids and friends with 7 seats, etc, and my feeling is that you could have the pick of them with your budget. Even $15k is not an unrealistic spend on one of these in a D2a facelift model, but I can't vouch for the condition of one at that price.
If you really must have a P38A, I too would not pay much more than $15k for the last of the 2001-2002 RR P38A. Those that are advertised in the $20k region have been for months.
