The fuel accumulator is with the fuel pump and fuel filter, underneath your car just behind the passenger rear tire.
Looks like this:
While it's possible that the fuel pump relay could be the problem, I would think that it would affect more than just hot starting. I don't have a 300E, but on my 300SEL, the fuel pump relay is in the shelf between the battery and the fuse box, under the hood. Looks like this: