If a surface grinder is out of the ballpark cash flow wise, a good etcher, more supplies (steel, belts, sandpaper), accessories for the grinder like small wheel attachments, better shop lighting, tongs for your oven, heat treat foil if you do stainless (and quench plates), a knife vise (this gets my second vote after the SG), a variable speed disk grinder (vote 3), and any number of other things would come before the mill and especially the other drill press. Not that those and a metal lathe wouldn't be handy to have...