I'm not aware of any underlayment that acts as a radiant barrier, not in residential roofing anyway. As for the other thing, you are referring to ice and water shield. That is a good product to have at the eaves and in the valleys. You wouldn't install ice and water shield and modified bitumen in the same area though. It's been awhile since I had quick access to code, but generally, slopes between 2/12 and 4/12 require double felt or an equivalent (ice and water shield), and slopes under 2/12 require modified bitumen or rolled roofing. I wouldn't bother with rolled roofing though, as mod bit is the preferred product. Again, I don't actually install roofs though.