A diesel with that many km may not be on its first turbo and if it is, it's been well looked after AND hasn't been used for towing. Problem is, it's hard to tell. I have a genuine Garrett in mine, and it's my 4th (although one of the turbos I bought was not faulty so should only be my third). Approaching 320,000km on mine.
The gearbox and diffs are likely to have suffered somewhat, especially if they have towed. There's a trick to them though, don't monkey around with "specialists". I took my car to a specialist gearbox and diff place, to look at my rear diff, they claimed it had damage in it (it was diagnosed with a weeping seal) and it cost me $3K to fix. I should have just gone to ARB and had a locker installed $2K.
The gearbox has now set me back $6500 and I've been warned it might cost another $5K to $6K to repair if there's any water damage (they supposedly found water in it). Very funny about that, it was checked a couple of weeks before and the oil was perfect, I've not driven in the rain since, no water crossings, and I've washed the car once (outside only, nothing under the bonnet). I smell a rat, but if there IS a problem, I'm buying a $500 gearbox off eBay and getting the local mechanic to install it, not some specialist.
You might also have wheel bearing issues, disc rotors ... you can spot these when driving it. Find a corner somewhere that you can cruise around at 60km/h or so. Do it one way and listen if the note of the wheels/tyres change. Now do it the other way and listen for any differences. If there are, the wheel bearing on the OUTSIDE of the corner is worn (so if you're turning right and hear the noise, it's the left wheel bearing).
There could be other issues. A motor that old should probably not have any trouble with its timing chain but a quick inspection inside the oil filler cap will give you an idea of the amount of sludge, which will give you some indication of the level of maintenance the vehicle's had. If the engine sounds ok, seems to accelerate ok (after the usual lag), doesn't have any problems changing gears and doesn't thump into gear, and when you get to 90km/h GENTLY there's no vibration felt as it locks its torque converter, then the car is probably ok.
Then it's an inspection for rust - look at the chassis particularly under the tub, where the rear wheels can really chuck muck around.
If it passes that test ... it's probably worth having.
