I would put some serious weight behind this. If you look at the price of unknown tdi engines on facebook/ebay they have gone up in price hugely and then they will need at least a full timing kit. I have just fitted a 200tdi to mine that I got from a friend who is into land rovers. He had removed it form his rotten disco about 10 years ago and put it in the back of the shed for one of his but never got around to using it. I trusted him when he said it was a good unit that had just been standing so gave it a light rebuild/check over:
lapped valves and new valve seals
new head gasket
new glow plugs
full timing kit and idlers
new t-seals
checked main bearings - fine and did not replace
resealed sump
new alternator
new water pump
new crank damper
new belts
If it was a completely unknown engine I would probably have done more work to it, but it was a trusted good unit. As mentioned above for the price and time and effort of getting a tdi fit to use you could do the same with the 6 cylinder. a tdi in my opion is no longer a straight fit as you no longer drive the vehicle home and remove the engine. You will be buying an palleted engine most likely untested. (also don't trust videos of them running. I friend bought a defender with a 2.5na fitted that did not run when buying but was assured it was a simple fix and shown a video of it running a couple of days ago. On investigation the timing belt had snapped!)
Do not confuse originality with condition or concourse looks. Body panels, as long as the correct panel, can be swapped with no consequence. This is the same with nearly all classics but the fundamentals need to be as originally specced and preferably numbers matching if you really want high value originality.
Some more information on why you thing a 6 cylinder would not be suitable as a daily might be helpful. I would expect mid teens as the mpg on it which is not bad considering its age. I get low twenties out of my series diesel and get 26mpg out of my 200tdi 110 that does 30k ish miles a year. they are never going to be good on fuel economy but you accept it costs a bit more to run because it will cost less in repairs than a modern car is far more interesting to drive. There are far more pressing issue with why a series vehicle is not suitable as a daily than mpg: elbow room, waterproofness (lack of), needing to fondle you passengers left knee to get first or reverse, the list goes on.
rebuild and reuse man!
but its your landy so good luck with whatever you go for 
