Play a bit with the item level entry until you found out out which one is the lowest for a tower shield to get the affixes. Subtract 5 and that's the character level at which you should go shopping for it. Optionally you can click on "magic items" and checkmark the "artisan's" and "deflecting" rows, to find out the chance to get a tower shield like that.
Tower shields have a maximum of 3 sockets at any item level, so that factor can be ignored.