Consistent way to sand and repaint doors?
Priming the doors will do the most to ensuring your previously stained doors have a consistent, even finish.  

I also like to lay a door flat and paint with a roller - makes the job a lot easier because you're not having to mask off each doorway, worry about runs and drips, etc to paint the door while it's hanging.  We didn't have enough space to set each door aside while it dried so we had to wait for each side to dry before we flipped the door over (and was only able to do one at a time), but they turned out really nicely so the extra time spent waiting was worthwhile.