I was hesitant to use panniers when I first started out. I didn't want to have to push them through the wind, my belly was enough resistance. I was lucky enough to work with a guy that was great with bikes. He said not to worry you already have two shins to break up the air. I already had a rack so it was just a choice of bags. I just got the best deal on what I could get.

They didn't make any difference I could tell in the pedaling effort at all. I don't know about one vs. two bags, I have two. When I don't have enough to use both of them I just split the load between them. The weight difference doesn't matter once you have a rack anyway.

My rack is just a rack......not the cheapest or most expensive. A mid-priced rack from Universal. I don't carry more than several bags of groceries and have never had a problem. I did use a seatpost clamp with mounting tabs instead of using the bosses on the seat stays. It just made for a nicer installation on my particular frame/rack combo.

You might do a search concerning racks for bikes with discs. I know nothing about the combination to offer any advice.