Suggestions for a social network website, how to create a web with buddypress, no multisite of post

I want to create a social network with buddypress, I need some suggestions about how to create it.
1- Which theme do you suggest?
2- Can I Customize the appearance of it? Do I need to change the css? Can I change a theme's css? In that case, How?
3- I want to personalize the questionary in the log in section, How can I do that?
4- I want to have tow kinds of users with different use of the site, one kind of user is free user, and second type of user is for offering their service to the rest of the users, this kind of user have to pay. How can I do that.
This web is more or less what I want to do:
http://www.erstenachhilfe.de/
Kind regards,
Alejandro