Well the current units are Ford (V12) and Jaguar (V8) derived and are built at a dedicated Aston section in the Ford plant in Cologne, so if Aston's need to buy in engines I could think of plenty worse than Merc's V8's to use.  Where else do they go?  It's got to be a big and lusty engine, and in reality (sadly) it probably needs to be German. I can't see that Aston's will ever go back to the old days of hand building V8's,  they only need 300 or so a year back then!  The cost of designing and building an engine from scratch must be huge in this day and age, what with all of the emissions regulations and the need for more and more bhp, so why not tweak an engine thats already very good?