With just a preamp and power amp running stereo isn't a big deal (and it's not really a big deal anyway,*)...most players that have racks run some effects that sound good in stereo (like chorus, delays, and reverb). .
Have you gotten the manual for the Peavey 50/50? I don't remember off the top of my head if you can bridge (use both sides of the amp as one amp) it to use as a mono amp...I know you were thinking of getting a 2x12", so the first cab you might want to get woud be a stereo 2x12" and latter you could get a second 2x12" to use (for small things use the stereo 2x12" and for big things, use the two 2x12"s in stereo).
*I'm a stereo junkie, so you don't know what it took to say that...