Does anyone have any special notes on buying pannier racks?
Bang for buck would suggest an aluminum rack, but these typically don't support high loads.  Once you start heading into the 20-30+ kg range steel performs better in terms of total load capacity and behavior under load (I.e., less flex which reduces the chance of a shimmy).  In terms of steel I have had good luck with Tubus and hear good things about Surly racks.  However neither of these racks are cheap. 

About fenders

In terms of frame eyelets and fenders, some racks (usually the higher end ones) have additional mount points for fender struts.  If you are handy you can often create your own mounting points.

About pulling building supplies

If your intent is to be in the transport building supplies such as plywood, you should really think about alternatives to racks such as a trailer. There are lots of different types of trailers. I've seen some flat bed like trailers for putting large awkward objects.