Do you get to keep the gear that you find in dead money after beating it? Can you return to the Sierra madre after completing the dlc?

You can keep everything you find, but you can't come back.
You have to let go.

Do you get to start out DM with some/all of the gear in your pack? With nothing? Or with just a specific number or weight total of items/weapons?

Nope. You get stripped of all your gear at the beginning, and have to re-equip yourself there.
But don't worry you'll get everything back at the end.

Thanks. I'll be sure to wear clean underwear.

:rofl:

I kept Moto Runner's helmet for some reason.