Hi Paul, the iTunes Store is the best value site for tracks, usually 79p or 99p per track, but you can often get full albums of DJ friendly mixes for less than £10, eg defected compilations etc.
I then use Traxsource as I’ve found it to be the best place for non cheesy dance music, there’s often 30% discount codes or at least 20% so you can get loads of tracks for good value.
As far as playlists go. Check out the many threads on here for advice but essentially you want to create a ‘music’ folder on your computer then use that as a base for all your tracks. You can then import them into your music collection on the engine desktop app, after that it’s a case of building playlists and adding music to them.
I personally set my music folders up to mirror my software playlists, that way when I add new music to each folder (genres in my case) I can then just drag and drop it over its playlist of the same name, I then know where everything is and I can easily delete and rebuild playlists if I ever need to.