Definately sounds like the fuel pump. I had a very similar problem with my 4.5 a few years ago. Had the pump tested by my local MB mechanic and it was truly the pump. Phoned Europarts and had the problem solved in a couple of days.
In my very humble opinion, when the ECU fails I think it tends to be catostrophic. The ECU is a big ticket item on that car and I find reprehensible that the mechanics would use replacement as a diagnostic tool. I don't mean to trash mechanics, as the majority are very ethical, but it's the small minority that makes it difficult for the rest.
I would find someone (Dealer etc) with the appropriate Bosch Tester and have a proper diagnosis performed on the vehicle. Within minutes of plugging the tester into the ECU the mechanic will be able to tell if it's the pump or not.