Studio Monitors. The cheapest kind that will get you good sound are the KRK Rokit series.
Here are the 5 inch monitors (the cheapest)
If you get bigger ones like the Rokit 6's or 8's you will get better sound but for the sake of cheapness I've provided you the link for the 5's.
Subwoofer. You need a sub to catch all the low-end that your monitors will miss.
Audio Interface. You need to be able to send the audio from your computer into both the sub and the monitors at the same time. The best one for the cheapest price is the Apogee duet 2.
Your interface will also allow you to record with a microphone into your DAW.
This is all very important shit. if you acquire this stuff your mixes will improve wayyyyy more and you will be able to further pursue that "pro" sound you want.
A midi keyboard is also very good to have.
Here is the Axiom 49 by M-Audio
Another good one is the Akai MPK 49