So, from what i could understand, i think i did the right thing.
I bought Dead Or Alive 5- Ultimate (not exactly, i still have to pay tomorrow, and it will take a while to get home).It is a new copy, so i don't have to buy online pass or something like that to play online, i believe.
Once i get the game, i'll follow the instructions on the Team Ninja web page to do the update to Last Round without problems. And then, i'll have Last Round, without the new stages (i wish i had that cool blue panels stage, but that is okay, once i get a good pc or a ps4 i'll buy again), and i'll be missing also all dlc costumes, and all doa 5 ultimate dlc characters, and the last round characters, and i'm fine with that, since my possible mains are Kasumi, Ayane, or Momiji (but i'm curious about Honoka, so i'll probably buy her in the future). But i'm fine with that.
Now, the online for this game on xbox live probably is just as empty as the online for Skullgirls on the 360, but at least i'll be able to practice and understand the game, and if i'm lucky on weekends find a couple of people to play.