I was going to put brakes on an old Demco Kar Kaddy a few years ago, then I found out that the spindles had no flange to mount the backing plate on. I gave up on that idea. I quit towing the PT Cruiser instead.
You might want to check to see what is required for your dolly to install brakes. It quite possibly might be easier to find a dolly with brakes instead of trying to install them on your old dolly.
As for drums or discs, I don't know which would be "best" in the mountains. Same goes for electric or surge. I have used both on various trailers in the past. They both worked just fine for me.