Both of these have my favorite gauge cluster for this era. My dad's '83 Chevrolet CE had it. Very nice pair!
I don't know from experience but according to what I read, by 1984 the Diesels were better. Though, the main issues these had (again, according to what I read and does this even apply to ones from the last two model years?) were the lame head bolts (stock ones were too weak) and lack of a water separator (important on Diesels.)
He's been selling those for a while now. I love them both but that's a rich price for the pair, you'd have to really be in love. I wouldn't fear the Olds diesel anymore. The water separator is easy enough to get from Racor or similar as the 6.2\6.5 guys have been doing this for years since the Stanadyne Model 80 filter base was discontinued and notorious for leaking anyways. The weak headbolts are an issue, but the water was a major killer (hydrolocking a diesel is bad news). The lack of replacement parts such as injectors is a real concern. I've debated doing a 6.2 swap, it's the same size as a BBC and the traditional Pontiac v8 fits in this chassis without issue, so a BBC shouldn't be too much work.
There are a few people who did swaps in these wagons, either from a 5.7 or a gas engine, to a 6.2 Diesel. Reportedly they get 30 MPG or higher on the highway. Here's one I linked below. http://www.thedieselpageforums.com/tdpforum/showthread.php?t=31829
I would love to buy that wagon. I've had Olds diesels for years and they are actually pretty impressive. 25-30 mpg in a 1980's land yacht is fine with me. With a few simple upgrades, and today's cleaner fuel and better oils, they do just fine. Plus if for some reason the 5.7 doesn't work out, you have a factory diesel car to drop a 6.2 or 6.5 into.
My '85 GMC Suburban with a 6.2 would do 19MPG at 75MPH with 3.73 gearing, so I would have no trouble believing that. It's one of those "want to do one day" things. Especially since 6.2s are so cheap and most of the public has no clue about them other than "bad news" they heard from their brother's girlfriend's cousin's mechanic once.
I love the idea. I'd change all the fuel lines to Viton though, since I prefer Biodiesel when I can get it. Biodiesel eats up rubber fuel lines but has more lubricity to it than ULSD, though it's strictly a summer fuel in higher concentrations. It doesn't have as much energy content but it also keeps the engine cleaner. Mechanic's best kept secret that's not a secret, but that they who aren't in the know think is dumb which makes it a "secret". Oh well.