I'm running a 5.56, 7.62x39 & 9mm upper all with the same stock Rock River Arms lower w/single stage trigger. The 9mm upper is a Rock River 9mm upper with a ramped bolt, I ordered the complete upper with the bolt already ramped from ADCO but any 9mm upper will do as long as the bolt is ramped, that will let it work with the stock hammer of the lower. For a mag block I'm using a Hahn top loader, it just drops right in the top of the magwell with no adjustment. And I replaced my stock buffer with a RRA 5.5 oz 9mm buffer. Total cost was $500 for the complete upper + $150 Hahn magwell block + $30 buffer. Since the magwell block just lifts right out I can easily swap between uppers at the range.
The disadvantages to my setup vs a dedicated rifle are that I can only run metalform mags if I want 100% reliability, they're about $40 each. And the hahn toploader doesn't lock the bolt open on last round, although other Hahn blocks will. Any of the bottom loading magwell blocks have some adjustment up & down which will let you tweek them to run the cheaper ASC/CProducts mags but they don't quick swap like the Hahn top loader does. ProMag also has a dedicated magwell block for $50 that seems to work well with their mags. I looked at the Oly upper & mag option but couldn't find a source for just an Oly 9mm upper & mag, only found complete rifles.