"note that this appears to be a Euro model"

From what I've read, the W116 SD is a US or North America only car. Not available even in Germany.

Motor mounts are not difficult to replace. Outside of the parts, you only need some hand tools (floor jack, piece of wood, 8mm hex bit on a long extension and 5mm hex key, sizes are approximate). I'd sure do it before a long trip.

Why not look for a used heater control valve? Who knows what else in the ACC system won't function if you bypass the valve?

Sixto
91 300SE
81 300SD