M103: long cranking time
 

'91 300E with the M103, about 144,000 miles (231,000 km). In the 7 months I've had it, I've replaced the head gasket, plugs, wires, cap, rotor, etc. It always took two tries to get it to start and it would always stumble and chug for a few seconds after starting, so I just replaced the fuel injectors and seals. Now it starts smoothly on the first try, but that first try consists of running the starter for about 2-3 seconds before it fires up. Is there anything I can do to get it to start more quickly?

You may want to replace the coil and fuel filter too.

Fuel pressure accumulator

Forgot to mention, I've done those too. But I'll look at the accumulator as duxthe1 suggested. I was hoping it wouldn't be that, because those things are pricey.

I don't think that 2-3 seconds crank is out of line for a M103.
The air flow sensor plate has to be drawn down before fuel is injected giving a delay unlike more modern electronic injection systems that start instantly.

If your accumulator was defective, the crank would be a lot longer that 2-3 seconds.

Also make sure none of your fuel lines are leaking or cracked. I had the same problem and just found my line leaking

Clean all fuses in the fuse box. Next job to do is all earth points, remove and clean , then grease them and refit after. Also check all vacuum lines for leaks ,the rubber conectors split .Then final thing would be , fuel aditive ,and a brisk drive.