Here is an inspection guide I wrote for general W123 diesel inspection. It uses a non-turbo sedan, some items will be different, but it should still help when shopping.
https://www.ifixit.com/Guide/Mercedes+W123+Diesel+Pre-Purchase+Inspection/21003
The only system that is completely different on the wagon is the SLS and you're getting some good advice on that already. Otherwise, the primary differences are just body parts, interior parts, glass and seals, stuff like that.
The 1981 model year wagon has some differences from the rest to be aware of. It was the first turbo W123 and it got some single model year parts before Mercedes figured everything out for the 1982 model year when all W123's got the turbo engine. For example, it uses an electronic speedometer borrowed from another chassis (W126?). All other W123's used the same mechanical speedo setup as your 240D. Many 1981 300TD's also were not equipped with a tachometer.