Could you take some detail pics of the rear seat sometime? mine has an adapted sedan seat and the universals had a split seat which i think I can make. ....Or use a W123 wagon seat. I also wonder what they did to the shock mounts. Mine are still sticking into the load space and yours look to have been altered so that they have been relocated somewhere else.
Being a 230S you can fit a OM617 easily in there although it may be more vualable with the original type of engine. You also need to rent a shed someplace to keep it out of that wet weather and work on it without upsetting your neighbors...