This is a very good question. And I'd say it is a bit of a mixed bag. I don't think such an engine will ever be worth the most. And I'd guess the majority of pre-Defenders where fitted with these over the V8. But just look at how crazy people go for an original engine Series these days, including the 2.25 diesel..... people actively spending quite a bit to reverse engine conversions. There was a time when you literally couldn't give a 2.25 away, petrol or diesel. You used to see complete engines on eBay for £25 unsold. Not these days. So I think this gives a strong hint to what will happen with these Ninety's, the question is just how many years before this happens.
Also worth noting, that an 88 model is only 4 years away from historic status too.
As to engine, the TD really isn't a bad engine. They don't like to be thrashed, which is what normally causes the problems. But drive them sensible and they should last. We have 3 Land Rover's on the farm with the 19j TD engines. All still run well and combined they somewhere in the region of 700,000 miles+
The one I use is an 89 County with only 152,000 miles on it. We did have a HG issue last year and a compression test confirmed one cylinder was down on compression too. Whipped the head off and found the expected cracked piston, so dropped the sump and pushed the piston out and replaced. New HG and it is running as sweet as ever.
Its a bit of grunt work, but if you are able to do it yourself (it isn't all that difficult, just a bit oily). It also cost very little in parts, £150 tops iirc.
Out on the open road you won't win many drag races and on high speed dual carriage way or motorway you don't have a lot of overtaking power. But in the hills/mountains the TD's feel surprisingly spritely and grunty. They'll easily pull sub 30mph in 5th an pull away in 2nd gear if you want. The TD is also noticeably more refined than a Tdi. Don't get me wrong, I love Tdi's too, but they are nosey gruff things. The TD, while still very much a diesel of its time, but is quite a bit more pleasant over a Tdi in the refinement stakes.
Ultimately, I'd say yes it probably is worth saving.
Tdi engines are all old too these days and will likely set you back £1000-1500+ to convert if doing it yourself. That I personally wouldn't go this option. Td5 or M57 would be the way, although both more costly and requires a gearbox change too. Or a swap to a Rover V8. But if all you want is a run around vehicle, there really is little wrong with the TD engine.