Say you buy DOA5U. This is effectively the most complete version of the game that exists right now. The only things it's missing are DLC videos (basically gravure clips of the girls), DLC costumes, and Marie Rose (and the announced future DLC characters). All the costumes, DLC or otherwise, from DOA5 and DOA5+ are either included in DOA5U out of the box or in the $19.99 Complete DOA5 Costumes set. The other costumes and costume sets were released after DOA5U, and should only be compatible with that version (and DOA5 Core Fighters, which is really just the F2P version of DOA5U).
Alternatively, you can download DOA5 Core Fighters, and then buy the pack mentioned in OP ($19.99). In that case, you're missing the story mode (available for $15, I think?), music tracks (somebody said $2, I'm not sure if that's for all of them or each individually), the DLC costumes specific to DOA5U, and Marie Rose and her DLC costumes, which you can purchase individually or bundled together.
The biggest problem is that there are too many options. You can buy a pack of all the costumes for a single character. You can buy a pack of all the costumes in a single release. You can buy costumes individually. And it doesn't track whether you already own the costumes from any of the other options. It doesn't help that you also have to download separate compatibility packs for the costumes you purchase.
The second biggest problem is that TK is being a massive dick about their DLC. I recognize DOA might not be a popular enough franchise to support itself without DLC, but the way they're handling costume DLC, along with their prices and the confusing nature of the releases, is pretty consumer unfriendly. This without even mentioning the fact that they themselves said that releasing DLC characters, especially for pay, was a terrible thing they'd never do when DOA5 released and now they're doing just that.
So in the end, I'm definitely holding out on DLC, at least, until I'm sure this isn't happening.
I was looking through the DLC on the 360 online marketplace (where I own DOA5U) earlier while logged in, and she was showing up as paid DLC. D<