To not burn the glow plug out you have one of two options:

#1: use a thermoswitch of some kind

#2 Use some resistance wire. Resistance wire is what's between each glow plug in a series system. Like this

Your design is very similar to others I've seen on the infopop veg. oil forum and the reports from it seem to be pretty good, you probably won't see any problems if you get it regulated somehow.

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'84 190d 2.2 - Silver
Converted to 5-Speed
'84 190d 2.2 - Blue
Factory 5-Speed

'76 300D (W115)
Sold, found again in less auspicious circumstances