Hiya BenzDriver,
1) Congrats on the new ride! Great cars. There are 4 in my family. (Can you tell we like them?)
2) 30mpg is excellent. The highest I've been able to get repeatably is 31.5-32.5mpg with a light foot. More typical is 28-30, and if you're getting 30, that is fantastic. Don't expect much more, even with disabling the EGR/ARV, but if you do get an improvement, woo-hoo!
3) Definitely disable or bypass the EGR and ARV. I removed all the junk from my car including the solenoids under the air cleaner, and the EGR valve itself. See my website link for photos of those, and many other, modifications to the 300D.
4) You are probably aware by now of the possibility of a bad head. Check what "number" your head is, and keep an eye on the "high cold pressure" symptom - as well as engine temps in the summer. Use ONLY the proper MB coolant and be religious about the 2-year change interval. The 1987 model especially, for some reason, don't tolerate poor cooling system maintenance. If the plastic coolant tank is old & yellow, consider replacing it - they are often swollen & cracked after 15+ years, and the new replacement (~$50) has an internal silica pack to control corrosion.
Best wishes,