I must agree with Dave.
I have the OM603 engine and I've been there. Two and a half failed (on the same trip) last winter and nearly disabled the car. Luckily the ones that had to be changed coincided with the ones I COULD change. It was cold and I had places to go. It was a huge pain.
Consequently when the shut-off valve failed a month later I had them change out the remaining GPs while the (intake) manifold was off.
Please carry spare (80amp) GP relay fuses with you. If you let the GP nut and wire twist to the side (easy when changing the GP under the manifold) and touch the engine -- poof there goes the fuse. It was a 45deg day when it happened and my car would NOT start without glow plugs.
'87 300TD 304kmi (RIP)
'95 Toyota Camry Wagon 125kmi