My 450SL will not start. When I turn the key and let go it continues to grind, but does not start. It's not the sound of a weak battery. Never seen this before.

It might be a bad starter.

If engine cranks but will not start then the engine is either not getting fuel (most likely on 107 SL's) or no spark or a timing issue. Easiest thing to check is to listen for the fuel pump (near right rear wheel). If it doesn't run, then probably you need a fuel pump relay. Found behind the glove box.

you may have a bad crank sensor check for a code if not check your fuel pump

whack the starter while turning (engaged) bendix gear engaged and stuck...also lube same...