I've just downloaded Dig Deeper, how scared should I be?
Bwahahahaha! Poor fool.
I've been playing since 2D. When I first installed Dig Deeper, I lost more forts over the course of one week than I had in the couples years prior without it.
That having been said, the mod is a lot of fun. Once you learn to survive, you'll probably never go back to vanilla.
Do orcs have any weaknesses?
Magma.
But if you want a more useful answer, my best successes with orcs have come from building retractable upward spike corridors. Make the corridor the only way into your fort, hook up every spike to a lever and when the orcs show up set the lever to auto-repeat. You may have to die horribly a couple times before you find a setup that works. Be sure to place the lever sufficiently far from the corridor, as you don't want the lever-puller to run in terror because an orc is half a screen away. Also note that orcs are inconsistent about opening locked doors. Sometimes they will. Other times they prefer to lounge around in the spike corridors. Note that with low quality wooden spikes, some of the tougher orcs will be able to stand in trap rooms for months out-healing the damage they take.
Do they attack earlier than goblins?
Usually. It varies. Most often I get orcs after about two seasons, but I've seen them take as long as two years before they show up.
What kind of stuff do they have?
Leather, iron and steel armor. Weapons of variable quality up to exceptional.
Is there anything I should know about minerals?
Nothing especially important comes to mind. But be aware that Dig Deeper introduces some new food types that the trade depot search screen has difficulty searching for. If you want to trade in food, be sure to produce standard food too, as if you exclusively make modded food, sometimes you'll have hundreds of food that you can't get to the trade depot.