20" of vacuum is sufficient. I would not recommend rebuilding the vacuum pump. I have a friend with a 1985 300CD who rebuilt his own pump (new bearings, etc.) and 3 weeks later his pump exploded internally and took out the timing chain, camshaft, camshaft towers, etc. essentially the whole engine. New vacuum pumps are around $350 and are well worth the money. I now change mine every 150,000 miles as a maintenance item. Cheap insurance.

The shut off valve on the ignition switch is cheap (less than $25) and easy to change.