I swear I'm not affiliated with the owner -- but I'm very familiar with this motor as my last obsession was '70s Benz W123s so I'd like to stick up for it and write you an essay.
As far as alternative diesels swaps the OM617 is a great option. Power is right there with the 300tdi. Reliability is absolutely epic. Not "sweet bro! a diesel! it'll run forever!" epic, but literally epic, as in tens of thousands of 617s across the world chattering away long past 300,000 miles as the rest of the car rots away around it. Drive more than an hour on any given day in a population center in the U.S. and I guarantee you'll see at least one or two 300TDs chugging down the road. Probably on its 6th owner whose dad gave it to his college daughter who sold it to her now ex-boyfriend and it hasn't seen fresh oil in 25,000 miles. That kind of epic. (You can say the same about Rovers, except that the MBs you're looking at are probably at least 10 years older.)
This truck here has been posted all over the American LR universe -- most people are trying to say it's "underpowered" and "too slow" and probably won't even hit 20 mpg. First point is that the OM617 isn't a turbo-charged OM616. Lotta people think that's the case but let's remember that when MB made the 617 they added a cylinder.
It's a turbo'd inline 5, not just a 616 (4 cyl) with a turbocharger.
Slow? Yes. You folks here have probably driven more 300tdis than most. The 300tdi isn't "quick" by any means. I have a very hard time believing the 300TD in a Disco would be appreciably slower off the line than a 300tdi, in stock or built form (truck wise). The HP/lb ft numbers are pretty darned close.
MPGs? Well, most Americans think the OM61x motors are magic 35 mpg machines and hold the motors to that standard no matter the application. The reality is they are incredibly overbuilt small diesels
put in incredibly overbuilt luxury cars (in that era) developed when the market was just beginning to see increases in crude oil prices. Most W123 wagons were pushing 4,000 pounds. I never got below 24 mpg in my last OM616, and got 25mpg at 70mph (much too fast for OE 4-speed). When I drove relatively carefully 27-29mpg was easy. Knock out a few for the 617 turbo and extra cylinder, the Disco weight/aerodynamic reduction, and then add a few for the taller R380 ratio and I think his claims are accurate. My best guess would 16-18ish pure city, 22+ pure highway, but 25mpg doesn't sound crazy if he sat at 55-60mph on flat interstate.
If I were looking at this truck my first question would be how he mounted the motor. My '95 DI 6.2L Detroit diesel uses stock (DII) motor mounts, a nice peace-of-mind when the opposite is wildly fabbed brackets and other creative solutions of dubious durability. The OM617 is a relatively small motor, smaller than a 300tdi (I think, I don't know 300tdis so well) and both are definitely smaller than my Detroit (which fits into the D1 engine bay like a stock V8), but I'd still like to see some pictures of the mounts. Someone mentioned oil pan clearance -- that's just the angle of the photo. The OM61x pan isn't deep and your diff will never touch it unless you're doing something very wrong.
Adapter plate he used is no issue -- anyone doing a swap can find a commercial piece or a template to make anything bolt to an R380 at this point. My 6.2 has been dropping 330 ft lbs to my R380 for 12,000 miles and I don't even have the 2nd gear grind that a lot of you V8/tdi folks have.
Tachometer is a common issue with these swaps -- there's no way to hook it up to the OEM gauge without getting seriously creative. Easiest way is to get a TinyTach or equivalent and splice the transducer into fuel line (easy peasy.) You'll still have a dead OE tach gauge but I have yet to see a way around that. Taking apart a TinyTach, soldering the circuit board to talk to a small 'lectric motor and using that to drive the OE tach? Now we're talking creative!
A/C? Easy to retrofit he says? HmmMMmMMMMMMmmmmmmm....
I have the same issue in my 6.2. Room to mount the compressor, etc, isn't an issue, and I have all the fittings, etc. ---- it's finding the belt to drive it! When he pulled the V8 and dropped the 617 in, what did he do with the 617 A/C pulley? He might've used it for the vacuum pump or something. The same case of too many systems to power/replicate but not enough pulley availability with the ultra-basic diesel motor that I'm dealing with. And you can't just add pulleys real quick. In order for me to free up a belt I'd have to convert my vacuum pump to a mechanical/gear driven unit. Not that hard, but it means moving other things, etc etc. and you know how that goes. Not a full-on hardcore job but I'd be skeptical about "Oh so easy just hook up original parts!" Although to be fair maybe the pulley is there and just waiting to be converted to serpentine. And another nice benefit of the relatively vacant Disco front clip is being able to mount an intercooler...
I'd want to know more about this if A/C was a requirement for me.
Mileage at 100,000 shouldn't throw anyone off. That's nothing. My 616 had 246,000mi and when my MB mechanic and I took the top end off for funizes it looked barely used, no sludge, nothing -- and that motor was built 5 years before I was born. Parts? Puh-leeze. MB build ten billion OM61x motors. The lack of fresh motors available shouldn't be an indicator of a lack of a parts market but an indicator that these freakin' things run forever if given even rudimentary maintenance. There's little demand for crate motors because you can't break these things.
What's the point of this ridiculously protracted essay? If the conversion has been done right that is a steal at $12k (if that's his suspected reserve, as someone said), considering a good 300tdi conversion will last you literally half as long and cost almost exactly twice as much. Hell I'd say even at $15k it'd be a pretty good deal assuming the rest of the truck was straight.
I really love this truck, wish I could go check it out.