Sorry for the delay! Your reply got buried and I only just noticed it :P
1. Koikatsu if you want more anime style, HS2 if you want a more "realistic" style. HS2 is their latest release so its toolset, lighting and character posing is better but it can't get that anime look that KK can.
2. Depending on how deep into the modding community you want to go it could end up astronomically high. For a good baseline I'd say have at least 50GB free.
3. I'm glad you've been enjoying it! :)