My experience has been that my D1's and Classics have been the most reliable Rover's I have owned. I have not had the chance to own an LR3 or MKIII Rangie yet but I have had 3 classics, 4 D1's a D2 and a P38. My P38 was easily the "nicest", drove the best, had the most power, looked the best. I had it converted to coils with a 2 inch lift, 285's and a Safety Devices roof rack, it was an 02 HSE so it had the 4.6 motor. I never had a problem with mine other than a leaking radiator which was a simple fix of replacing it. I sold it last year and have regretting it, however the current owner has had nothing but trouble with it. He filled the tank with bad gas and plugged his injectors and ruined the fuel pump, and then he got the dreaded Open Book symbol on the dash to the tune of 1500 for new blend door motors (I think that this is more likely to happen if the system is left on auto mode)
I'd say buy the P38 if you like the style, find a good one and enjoy it! There are work arounds for almost any problem you could have and you get the bonus factor of driving a car that is unique and fantastic to drive