music converting format site

I want a site done (probably with wordpress) that will allow a user to login with their free spotify account and search for songs/artists/albums (whatever spotify api allows). The user then can click to download the song. The site will have to be able to convert the spotify format into mp3 so it can be saved on the users device. There are several desktop software that can currently do this. Might be easier to convert one of these to web based? Alternatively, if the user does not have a spotify account, they can sign up on my site and use mine will be a paid one if needed). Only members will be able to use the sites spotify account. Registered members on my site will also have the advantage of not seeing ads. Obviously the members won't be able to see the login details, so the site will have to handle the login to spotify. The back end will have to handle the 2 types of memberships(guests who use their own spotify and members using sites spotify), Guests will be subject to ads. Members won't. So the backend will also have to handle the ads. The homepage will be very simple. Login with your spotify account, or sign in as a member. IF they login with their spotify account, then all searchs and such would be done with their account info (I assume that is needed for api pulls?) IF they use the sites account, then obviously the site would use its own info to pull searches. In both cases, when the user selects download track, the site must download from spotify and then convert to mp3. I am open to suggestions