Seat cover pieces are available. Try PartsShop first. World Upholstery and Performance Products have them also. The MB Tex is very true to color and you can just do one section if you want and have a good match. I just finished my 300SD this weekend. Remove seats from car (CD has a reasonably decent description) and go to it. An auxilliary power source (motorcycle battery) is an advantage for moving the seat back and forth on the rails as you get to first one hidden screw then another, after it's out of the car. For sure get new horsehair pads (for appropriate seat or back cushion). The spring frame can be replaced ($$) or two tennis balls in the front coils of the seat springs worked great for me. Some plain cording crammed in the skirt channel of the seat cushion is a neat trick to keep the skirt from popping out of it's channel. You'll know what I mean when you get there. Good luck, it really isn't too bad a task, but you need a strong back if you have to move the seats into and out of the car ten times. Don't ask how I know.
The Golden Rule
1984 300SD (bought new, sold it in 1988, bought it back 13 yrs. later)