Well, a P0087 is an OBD-generic code for "fuel rail pressure too low" ...
But a no-start condition can also be caused by the crankshaft or camshaft position sensor not reporting
Your test of "no fuel return" hints that the fuel valving solenoid (at the high pressure pump, i think) may not be opening ... but
that can be caused by the ECU deciding "if i can't see the crank/camshaft, i'm not going to allow fuel"
(but without walking thru the manuals, i don't know "which comes first: fuel pressure or shaft-sensing?")
Full 2003 and 2006 service manuals are available at:
http://aie-services-2.net/Sprinter/
Those cover the what/where/how of the systems.
On the off-chance that the 2003 manual set doesn't include one, here is the 2004 Powertrain diagnostic manual:
View attachment 2004-VA-Powertrain.pdf
...which covers the "try this, then this, then look for this, then this...." sequences
(you'll note that "P0087" isn't listed, since it's an OBD message, not an MB error code)
good luck
--dick