The skin after radiation can have much less stretch and be more fragile so normally if planning on an implant reconstruction they normally would want the tissue expander in and well inflated prior radiation I believe. You could have your expander put in your affected breast pre radiation and then still go ahead with the other one at a later date if you wish as this side will not be affected
I believe also that radiation still causes a lot of fatigue...but less so than chemo, but as you may still be recovering it is difficult to know how it will impact on you.
Hope this helps a bit. Kath x