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These cars hold water just like the A-body versions.
Look for rot / rust at the trunk opening & lid. On these cars there is a rubber seal that goes around the upper edge of the entire opening and come together right in front of the lock area. When you're washing the car / raining water has a tendency to sit there, which can cause rotting. And if the rubber seal is bad the water just floods into the trunk and channels. The trunk lid can hold water at the bottom edges / corners.
Look around the windshield trim, drip rails and rear window trim for signs of rust too. When washing I usally go over EVERYTHING to make sure it's dry, yes it takes an extra ten minutes or so both well worth it.
Also check the grille mounting areas for cracking. You'll have to open the hood for this but look behind the header panel between it & the core support. You'll be looking for cracking i the post. A quick-n-dirty check is to gently push on the outside of each grille near the middle / bottom, release and see if it snaps back quickly - if it doesn't snap back or lazily comes back one or more of the mount are broken.
If it has the rally gauges they can be lazy, this is easily repaired by removing the cluster and cleaning the traces / connection.
Be aware if it's a power antenna car it's in the passenger-front fender. You can get too it without entirely removing the fender but it's a PITA. Better off just removing and go from there.
If this has a console be careful with the release button, it's not the best design and the tang can break easily. Most people just yank up on the from the front (it's swings open from the passenger side) then wham, broken button. Also, the inside of the console can be lighted so check that as well (if lighted there's what looks like a white dot on the back wall of the console).
Sagging headliners are a common problem on these.
The transmission is a 700R-4, alternate you can go with a 200-4R. Depending on which you go with save some $$$ for beefing them up if you're planning any decent amount of power. Each has it's advantages / disadvantages, just check the engine swap and drivetrain threads. Investment in crossmembers, TV cable, linkages and the like may be needed.
Try to invest in a chassis service manual. If you get a Haynes don't expect much of info other than very, very basic stuff like fluid changes, light bulb replacement.
Check the RPO codes too, there should be a tag on the inner trunk lid. Stuff like F41, G80 are good.
Good luck & keep us informed.