I probably misunderstood you. I thought your priority is in composition, seeing the previous thread on games soundtracks. If your priority is to go into synth and synth playing technique, then it is a slightly different ballgame. Even then, there are many types - do a search in previous posts. There are lots of discussions on workstations, digital pianos, synths, midi controllers etc.
If you want to go into synth playing, be very careful. You still have yet to complete your piano exams - playing too much synth at this stage will affect your finger techniques, especially with unweighted keys. Even the weighed ones will affect you, since it is not the same as the real piano keys.
Now I see what you've been using the sequence your music! With a mouse and PC, the music you sequenced is phenomenal! With a keyboard, you'll be able to achieve much more. The mod wheels and pedals in GPO are very easy to use - pedals triggers automatic legato, mod wheels crossfades between different articulations and expressions. Read it here:
http://www.garritan.com/GPO-control.html. I promise you with a mod wheel and pedal, you'll create amazing music. The Jazz and Big Band collection from Garritan is the next coming library from the same producer (Garritan). Includes saxophones, rhythms and jazz brass playing styles. A good complement to you collection. But please, please... don't use the mouse for sequencing!!!!