Hi, You can buy the optical drive, use a USB one, or use a USB memory stick for the OS install. There should not be a problem with upgrading the CPU later.   
I am considering a Poweredge T20(for home use).  Now,I see it doesn't come with a DVD Drive,which is a slim drive,I see,so is there an option during purchase to add one,or to purchase one after? I guess I could always install an OS via USB Flash Drive.  I see in another post that  there is one motherboard config only,so that said,I could buy the G3220 now and upgrade to the Xeon later?   
T20 configuration questions