I looked at the back seats again - they don't look so bad. I didn't see rips or tears. Get yourself some Simple Green and a soft scrub brush, and scrub them clean with a mild solution. Unless they were re-covered with leather, the material is the fake stuff. If the color fades, you can buy ready-made cans of dye from World Upholstery (worlduph.com, I think). I've used them for three Benz restorations now, to include dye.
If the seats are saggy, new pads are available from the same folks, and the replacement is deceptively easy. Do some general searching in this forum for the same subject and you'll find some good information on doing such. If you don't want to buy new seat internals, you can use old blankets (for my W114, I went to a second hand shop and bought a bunch of blankets for a few bucks)
As for carbs, I'm way out of my league. The gas tank though, unless rusted through, can easily be saved. Take it to a radiator shop and have it steam cleaned and recoated. It's a common job.