I've been messing with a DD Build for a bit now since our Linkshell is pretty much set on tanks. You pretty much want anything that will boost your STR or overall Attack power, like the Bladed Lantern Shield does. The Vintage Haub is the best DD GLA gear you can get currently for body, with Cavalry Haub being second. I use Iron Gauntlets, since no hand parts give you a STR boost, and I use Iron Sabatons because more HP is better than +1 STR.

If you have good Dex, or generally hit enough, you would likely want to get 4 Sunstone Rings, or 3 Garnet. I use Sunstone because I use 2 of them and then 2 Silver Rings for more Acc. The Helm is really your preference. If you want more Dex, go for a Coif, or a Celata for STR and HP. I personally use the Visored Barbut for the look.