If you were looking to get a Fantom X6, you will probably want something like Komplete 9 Ultimate or Omnisphere in addition to Sonar. At the cost of a hefty chunk of hard drive space, these will give you the well-rounded bread and butter sounds collection you're looking for, in high quality. However, there is a very decent selection of basic sounds in Sonar X3 Producer. Addictive Drums especially is great, and you can turn to Dimension Pro for a broad variety of sounds. Dimension's sounds won't match what Kontakt or Omnisphere can deliver, but you could definitely get by with them and wait for a nice sale on either of those. The included synths like Z3TA and Rapture are already beyond what you'll get on the Fantom. I also found the SI-Strings instrument (acoustic string ensemble) quite useful, if limited in its range of sounds.
I still play a keyboard live (Korg Kronos 88) and prefer the coherence of sound and quick sound changing/setting up I get with a decicated board over a laptop based solution (which I've done). But in the studio, for songwriting or recording, definitely a computer based DAW.
You'll need a soundcard/interface too though! Something basic will do I suppose, as long as you can plug in your mic and guitar. Still, it will add another 150/200 to the budget. The Focusrite Scarlett series seems to be quite popular on this board, I seem to hear nothing but good things about them.
Also, your computer needs to be at least semi recent (i5 or i7 based would be best), with at least 4GB (hopefully 8) of RAM, and not completely filled with junk.