You can get a set of MB-TEX front seat covers for $450 on eBay. If you check the 35 year old used ones, they all have micro cracks that become more apparent with usage and sun exposure. There are a number of weak points on the back seats also that just get worse over time. I realize $450 is more expensive than a used part, but seriously why bother if it's just going to fall apart again in a few years?

https://www.ebay.com/itm/MERCEDES-BENZ-W123-SEAT-COVERS-240D-250-280-E-CE-TE-300D-CD-TD-1976-1985/180179993854?hash=item29f39084fe%3Ag%3AstMAAOxy7RBRY8t3&LH_ItemCondition=3

Dkr.