You’ve hit the nail on the head - I don’t want a stock original car that I won’t use.
I want something that looks stock but, is slightly easier to live with on a day to day basis.
My suggestion for a 200/300tdi was mainly due to them being plentiful and, in comparison to the original 2.0 lump, it would be quite ‘modern’.
I’d like diesel as opposed to petrol so, going back to the original question, how do I do it?
Can anyone point me in the direction of someone who’s done it?
I'm not sure what Series 1 you have, you mentioned lwb, is it a 107?
As for fitting a modern diesel. How much money are you looking at spending? And how much are you looking at doing yourself? And how bespoke? Are you happy spending the money/effort on one off items?
Firstly I'd have to ask, why diesel? How many miles do you think you'll be doing a year in it? If it's say 5000 miles a year, the £/mile fuel cost difference really won't be great between a diesel and a V8.
Tdi engines are fine engines, and will fit in an S1 pretty much the same way they do in a Series II or III. But they will be loud. And as pointed out, they are not modern. I personally wouldn't do this, and this comes from someone who has a Tdi converted Series III 88.
Other questions you need to consider. How much "Rover" do you want to retain? If you want a Rover/Land Rover engine, then you have the L-Series as used in the Freelander and Rover cars. It's actually a very nice engine and fairly modern.
The Td5 engine, which is very nice. And then there are some BMW based engines such as the M57 found in the L322 Range Rover. But fitting these will require considerably more work and effort.
Engine swaps may also mean a transmission swap, as not everything will mate up to everything else. Unless you plan to commission a custom bellhousing or adapter plate. And then you'll have to look at the transfer box also. If you go LT230 you may need to convert it rwd too, as you won't want to run the front axle in 4wd on the road due to the UJ's rather than CV's.
Which then asks, are you going to use different axles too?
Ultimately almost anything is possible. You could take the engine and drivetrain from a Jeep Cherokee (nice 3.0 litre Merc diesel) and plop it into the Land Rover, but it would require significant modification to make it all work.
If you are wanting things like power steering as well, this is all possible, and there are lots of potential option.