I would go to the junkyard and get a used set of calipers and rebuild them. I prefer ATE calipers because you can get rebuild kits for them from this website. It's kind of "steppy" but satisfying because you avoid the well-documented problems with rebuilt calipers not fitting right and you don't run the risk of disabling your ride waiting for the right parts to come in.

I use Dow Corning High-vacuum silicone grease to lubricate the pistons for re-assembly.

I've read about upgrading to W126 rotors and calipers that might also be something to think about.

I kind of remember reading that wagons have thicker calipers in the back - the w126 calipers are also thicker so that shouldn't be a problem. Gen2 w126 (86 to 91) uses a larger wheel - that might be a cool mod, too.