Does anyone have any special notes on buying pannier racks? I have a 520 mm cro-mo vintage tsunado road bike. It has 27 inch wheels. I'm putting a pannier rack on the back *preferably steel, but I'll consider Al.  It doesn't have the upper mounts but the dropouts have fenders so there are points there. I am going to be using it for touring, commuting, even picking up materials for the construction I'm doing (if anyone knows how to load a 3/4 inch sheet of ply onto bike,  I'll be really surprised)  I'm sure that I'd like to be able to load at least 25kg on that.

I'd be very interested in any pointers on what to get.  I'd like to keep the price in the best bang for buck range,  function/quality nothing fancy. If there any thoughts on bags I'm equally interested - Thanks
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.