Always worth spending the money for external audio/midi interfaces. Look at Focusrite gear its all really good stuff and used from iPad interfaces all the way up to world famous studios. A basic one will give you multiple outputs for speakers with software to configure them to quickly switch and headphone outputs with dedicated volume controls. (this doubles as a ADC/DAC) They also have really good quality pre amps meaning if you ever get a vocalist in to record some loops or anything like that you already have the gear for it. Monitors are also a great option, KRK's are good but have an "unrealistic" bass response meaning although your mix may sound great through them when you take it anywhere else it may not have the low end that you had when you mixed it. Look into the new or previous generation Tannoy Reveal series for $200-$300 you can get a really nice pair of monitors. Another thing worth looking into is where you position yourself in a room to mix if you are using speakers, things sound very different if you are right up against a wall and extremely different if you are sat in a corner. You can have the best equipment imaginable but if you don't set it up in the right way you could be wasting your money!