Stalling and Engine start up problem

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hi guys! i have a '03 3.0 diesel and i'm very happy with it until about a month ago. night time, while on my way to the city travelling at around 60mph, i heard a BEEP and computer showed FUEL INJECTION SYSTEM followed by the engine stalling. fuel gauge level was a little above a quarter tank. as per advice of BMW mechanics, i added some 5 liters of diesel but it still wouldn't start. on monday, computer diagnostics showed that it wasnt getting enough pressure from something in the fuel system. they checked everything and after 2 days, they told me that air probably went inside the fuel pump or something and that caused the stall. they released it to me and asked me to keep observing it especially when fuel level hits below half coz they're thinking that gauge or floaters might also be the culprits. i've had it 4 times almost empty tank and had no problem until 6 days ago. it cranks but just wouldnt start. now they tell me that its absolutely the primary fuel pump that needs to be replaced (part# 16126756157). it's a bit disappointing coz i feel they're not really sure. it's quite expensive so i would just like to hear any comments from you guys before i have it done. thank you and more BMW power!