Rust prone areas are inside lip of rear wheel arches(watch your hands when checking, you don't want to be picking rusty splinters out of your pinkies). Also bottom of doors.
Ask the seller what mileage the oil and filter is changed (should be every 4k max).
Ask them not to have the engine pre-warmed before you get there. If it's cold and she fires up easily then the glow-plugs are ok (although this is not a big issue if it needs new glow-plugs).
Another issue with it needing a fair bit of cranking to fire up if fuel range showing around 175 miles or less is the submersible fuel pump (again, not a major issue, usually just needs cleaning).
Look at tyre wear, if it's nice an even..happy days, as long as they're not low mileage tyres. Wear on the outside of fronts is usually tracking, wear on the inside is usually the camber that needs adjusting. A full geometric will sort both and is well worth it.
Properly set up the Mig drives well for a big old barge and steers like it's on rails.
Take it for a run to warm it up and then give it a generous revving to clear out the exhaust. Then ask the seller to give it a few revs whilst you watch the tail pipe. Should give very little black smoke unless revved hard. If it's got a recent MOT then it should be ok due to the emissions test anyway.
I have a '94 2.5td auto with 225k on the clock and the engine is sweet. My manual '00 has about 103k with no issues there either. If I had to choose between the two, I'd say the auto had a slightly sweeter motor, even with it being 6 years older and over 120k more under its belt.
How much filthy lucra are they asking ?