Regarding individual costumes purchases, the ability was fully added to the PC release in all regions. Currently on the PS4, only Asian regions can purchase individual costumes, except outfits for Kula & Mai, which are available individually in North America & Europe.
Team Ninja nor Koei Tecmo have commented on whether, going forward, individual costume purchases will be available for all characters on the PS4 in North American & Europe or if Mai & Kula are just an exception, due to the characters not being retroactively included in the previous costume packs.