1- The trap oxidizer was used on the 1985 300D in California only and mounds UNDER the turbo. That is the big silver can you see. Mercedes will replace it free if it hasn't been done already. It is replaced with an oxidation catalyst (different can). They will not replace the turbo. On 1986/87 models (with a totally different engine) the trap was mounted on TOP of the turbo, and if it failed it could damage the turbocharger itself. If that happened, MB would replace the turbo under warranty. On the 1985 models it's not possible for a trap failure to damage the turbo - so don't lose sleep over this!
2- The air recirculation valve is on the turbo itself, not the intake manifold. It is installed on 1984 CA models and all 1985 models (CA and 49-state "Federal"). It's purpose is to enhance the operation of the EGR and trap oxidizer by reducing boost pressure under certain conditions. You want to disconnect and plug the lines to BOTH the EGR valve as well as the air recirc valve.
3- There are a LOT of differences on the 1985 300D compared to the 1984 and older 300D's. The tranny is different, torque converter, rear axle ratios, more electronic widgets, and all sort of misc. items. Mostly they're all GOOD things, as it was the last year the car was made and MB was putting some new technology items on that were used in the 86-up W124's. Nice car!
Regards,