I believe you can still download them from the marketplace. There's a service called "Xbox Live Free" (formerly "Silver"). The Xbox.com landing page indicates you can purchase DLC with it, but you still need to sign up if you haven't already. I remember the in-game Marketplace not populating with anything when I tried it when the first content pack launched, so I've just gone to the marketplace.

Whether the costumes are worth it just depends on your own view. At this point there are 7 content packs, 1 of which is free. If you purchase the remainder as "packs" you'll be spending an additional $63 on the game. The general community sentiment is that the prices are exorbitant, but don't let that stop you if you like the content. Team Ninja hasn't said anything about a "complete" or "gold" edition, but the DLC should be permanently available, so your best bet is probably to look at the previews on the offical DOA5 site and just choose those packs that strike your fancy.