The Blackburn exp rack is a tried and trusted piece of kit capable of carrying heavy loads. The only thing it lacks is a welded rear bracket for bolting on an LED lamp. Tubus racks are another brand worth looking at for heavy loadcarrying.
The features you need in a heavy duty pannier are:
-Tough material. Waterproofness is hard to find. Weled polyester such are Ortleib are totally proof. Canvas material such as Carradice are highly resistant. Coated nylon may be waterproof to start with but it rarely lasts more than a few seasons before leaking.
-Lightweight stiffening boards in side and base. You dont need heavy material, corrugated plastic is ideal.
-Flap top closure for occasional overloading. Zips can break when stressed and roll-tops are harder to get into for everyday commuting, they are more for touring.
-Clip-on locking mounting system. Hook and elastic systems are nopt recomended, they are slow to mount and less secure.
-Heel cutout for forward mounting on the rack. Square profile shapes need to be mounted further back to avoid heel strike, and affect the balance when weight is behind the rear axle.