Hi Joel,
There is a good summary of what to look for at this URL. Although it doesn't state it on the page, it was written specifically about the 1987 300D/TD:
http://www.mbz.org/info/articles/misc/used124/
There were only a few of these made, something like 3000 sedans and 1000 wagons in 1987 - relatively small numbers compared to other MB models. The wagons tend to be pricey because they are rare, and most are pretty worn out by previous owners. 138k is low miles, and like surfblau said, price can vary a lot... could be over $10k if in REALLY perfect condition. Fuel economy varies widely, partially due to fuel used, but typical numbers for a good-running '87 are 23-25mpg city and 28-30mpg freeway, possibly higher if you have a really light foot. Most of the quirks should be mentioned at the URL above. The only thing I noticed missing was the vacuum pump - this should be changed if it hasn't been done yet, at almost 150k it's due (original pump had design flaw, new ones shouldn't fail.)
Good luck with your search,