Okay - V8 D1 owner here. Mines a 3.9 manual, but anyway.
Power - excellent. Will blow any 300tdi out of the water, even a modified one, and even if you fill your boot with lead. And it makes more torque than a 300tdi, and the clutch is much lighter. Also because of the gearing etc, its a much easier and more relaxing car to drive, it hits 50mph in 2nd gear, but has ample power from about 35mph in 5th. Even the steepest hill with the heaviest trailer, 60mph will be maintained, even if it takes 3rd gear to do it. Obviously the auto isnt as good.
Fuel consmption - awful. I would average about 14.5mpg over a tank. On a long run, if I didnt use my right foot too much, Id get almost 18mpg, almost. Around town, I got 12mpg. Over a whole 20 gallon tank.
Reliability - generally, they are quite mechanically reliable. If you overheat them, theyll go wrong, but then thats the case for most engines. The problems come when the computer says no. Mine broke ages ago, it went to 2 garages, neither of whom could fix it, and Ive lost all motivation. Its basically a giant paperweight, but its not worth much as scrap, so theres no point in selling it, I might get round to it yet!
A few other things, depending on when your particular car was registered - if its 1993 or newer, it needs catalytic converters to pass an MoT, and that means you cant run carbs, which means you need the EFI, you can use LRs original one, or you can change to a megasquirt or whatever.
Differences between the engines are minor, but basically, the original engine was a 3.5, and to go up to 3.9 the bore size was increased, and that leaves the block a little weaker, so it can crack, and/or the cylinder liners can drop, but from what I understand, this normally only happens when it overheats. The bores for all the engines bigger than 3.5L are the same (afaik) so the only difference is the stroke, which is determined by the crankshaft. I dont think you can put a 4.6 crank in a 3.9 block, but I could well be wrong.
I have an interesting proposition for you. I bought a 1999 3.9L V8 auto D1 as a running, driving parts car for my manual one. I think its on 155k miles, the bodywork is awful, the interior is pretty grim, and Ive not even bothered looking for rust, but theres probably plenty. Now, currently it doesnt start, because the battery is flat and the wire to the starter is buggered, its also sat for a fair few months, as I won it on ebay sometime last year, and it hasnt moved since. So here is my offer, should you choose to accept it - if you can fix my manual disco (i.e, get it started and running for a sufficiently long period of time, such that it is definately fixed), you can have the auto one for free. Now that may seem like a great deal, free car for some tinkering, or it may seem like a **** deal, several hours of your time for something thats worth a few hundered quid, thats up to you to decide. Also, it goes without saying, if you cant fix it, youve wasted your time and you dont get the other one.
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