You will want panniers with a solid side so they cannot get into the spokes of the wheel. Measure how much distance you need from your heels to the panniers to determine maximum size that will work.

Load capacity means next to nothing. What is important is how rigid the rack is with a load on it and want to minimize any sway of the rack and its load. The touring type racks have bracing to insure there is zero flex. They are not designed to fold for shipping which also makes them sturdier and better overall. Check out the design of the Blackburn Outpost Rear World Touring Rack. It is made much like the ones I have had to buy from UK dealers in the past for my own touring.