1) Yes, PhotoPost has its own datbase layout.
2) Yes, it is easy. You can use the built-in stylesheet editor to change the colors to match your site.
3) Linking to? You mean putting a link from your current page to PhotoPost?
4) That depends on you - PhotoPost installation probably takes about 15-20 minutes if you have all the pieces installed (imaging program, etc).
When you say "integrated" into your existing webpage, I'm not entirely sure I understand what you are saying. PhotoPost becomes an extension to your site providing a gallery - you would link to your galleries from your existing webpage.
Please do not PM me for support or sales questions. Thank you for your understanding.