It all depends on how much work you want to do yourself.
Here are some pointers...
1) Ask for service records - these are key
2) Check the glow plugs. If you see one thick wavy wire going from plug to plug, you have the old in-series plugs. You should have them changed out to the newer in -parallel wired plugs and relay. These have individual wires going to each plug - sort of like the spark plug wires in a gas engine.
3) Check for rust - Floor Pans, Rocker panels, around the headlights, etc
4) start the engine when it is cold. In other words if the seller has "warmed it up for you" it probably has a cold starting issue - could be the glow plugs, valve timing or loss of compression...
5) These cars are NOT going to be fast as this is a four cylinder non-turbo. I THINK it has something like 57 HP. If it is an automatic it will be slow - glacially slow.
6) If it has been running veggie oil and has not had a decent conversion - heated fuel lines, two tanks for starting on regular Diesel fuel, etc - run away
The good thing is that these are fairly simple cars and can be very reliable - I mean this one is 38 year old and has not been parted out...
If you want to get dirty and do things yourself these can be very fun to work on. I look at my old Mercedes as therapy
This forum has loads of searchable information and lots of good people who are willing to listen and help
Best of luck!