I've had the RR classic, P38 and a Disco v8. Out of the 3 of them I'd have the Disco v8 any day. Many many faults on the RRovers & I had the P38 4.6 too so I know what I'm talkin about with them. The 4.6 engine is WORSE than the 4.0 litre as it was bored out more. This caused the cylinder walls to crack due to not much meat being between them & the water jackets. They over-heat and then cook the cylinder heads. Having the liners replaced and various other fixes that can be done will prolong the engine life (and is expensive), but in essence once it goes, sadly its better having a brand new unit (see RPI engineering ont interweb).
My heads started leaking water at the rear (nearest the bulk head area). You could see it pouring down the block once the engine was warm.

Air-suspension

great drive when it works, right PITA

once it starts playin up. There's a lot that can go wrong with it thats why so many owners rip it out & replace it with springs (Really spoils the drive though). If you look at the ends of the rubber bladders if they seem perished they will start to leak and want replacing soon. Hight ride sensors are another PITA, as are the compressors

. In the end I ripped the whole lot out and started again, new bags, sensors, compressor, valve block etc... as it always seemed as soon as I'd fixed one problem another one raised its bloody head.

Then there's the air-con!!!! HEVAC systems have a mind of their own when they start getting elderly!!! Many owners have had problems with the temperature sensors saying one thing and it being totally opposite??? If the little book on the HEVAC system is flashing get your wallet out again
Also check the headlining & sunroof for leaks. Blocked scuttle panels can wreek havoc with water ingress & with all the electricery in a P38 water is not a friend!!!
After saying all of that, I actually loved it when it worked

I know I've slagged it off somewhat, but when they run well nothing can beat em. Your high up, sat in an a comfy armchair and waft around in luxury. Offroad capability is second to none & even an elderly P38 can show the newer X5's & such a thing or two
I was lucky enough to be able to do the majority of repairs needed to mine myself, but if you can't its roughly £80 at dealers just for a diagnostic check
Mine was LPG'd & it bloomin needed to be, MPG is only about 16ish if you tickle the gas pedal with a feather

raises to about 19-20 on a run. There's quite a few dedicated sites that show how to's just like this one, I used this one a lot Rangerovers.net its very informative and the problems and fixes guides are brill.
For reliability (and I use the term loosely) my Disco V8 was a lot better. No Air-suspension, smaller better engine 3.9l and not as much electricery to go wrong. But even that had its moments.