Ethically? Legally? I've already started building the front-end and it's pretty much complete. Even though the theme/idea of the site will be totally different, should I be having second thoughts?
A lot of people heavily borrow from existing site's designs; however, it'll probably hurt your portfolio. If people realize that your designs aren't actually original they may not want to hire you or they may insist on reduced pricing.
You should take significant inspiration from other people, as long as you re-create it yourself and change it in a meaningful way.