I have two, both '86s. Look for:
-little or no rust-anywhere, like where the front thrust rods bolt to body. Pull up the rear window gasket at the bottom outside corners and look there. Under battery tray and same place on the driver's side. In (under) the chrome center strip in the front bumper. In the trunk lid and around the taillights. Bottom of doors. Pull up the floor mats and padding. Have a good look at the fuel and brake lines. Under/behind the plastic body molding/cladding.
-#20 or better head. The '86 models seemed to last a bit longer since they did not come with the dreader trap oxidizer. They did however come with a #14 head and eventually they do all seem to crack. Also run the engine and look for liquid bubbling up arouind the injectors at the head. Usually means cracked prechambers. I'm just living with a bad one at this point.
-rubber front suspension parts have been replaced. There is a lot of rubber up there to give a nice ride I guess. it doesn't last forever. Most of it is not terrible to replace, just bite the bullet and buy the real MB parts. Become good friends with the parts manager at the dealership or shop other dealerships that advertise. MB of Westchester in Cincinnati will cut deals for MB club members and one of the guys there (Mike)is a 126 nut case like me. Also Tom Hanson at the classic center can get almost anything you need and is one of the best guys I have ever dealt with.
-Ball joints. Should be fairly new. They last awhile but are a bit of a pain if you don't have the magic tool ($$$$) to remove/replace them. I have done several sets by substituting (cubic) curse words for the magic tool.
-Rear axle shafts should not click or make any noise (listen taking off or in slow turns).
- rear lower control arms can rust. Check for rust hole in the bottom. Can be replaced with parts from salvage yard.
-Interior will probably be toast in the south and hot west. Should be able to replace from a rusted model from up north.
-Odometers fail (little blasted plastic gears), so verify mileage with receipts or such.
-All that said, for a long daily comute, a 126 is a great way to go. Quiet, very comfortable ride that will leave you rested when you arrive. If the cruise control works, even more so. 23-27 mpg was my experience at 70 mph. They seem to have a sweet spot for mileage so a little careful experimentation may find the best for you. One SDL I had seemed to do better at 80 vs 70. Made no sense at all but I was driving long distance several time a week when mom was sick and got a chance to check.
For $5000, you should be able to really pick and choose.