Hi JL,
I'm neither a Sonar user (I use Cubase), nor a Casio WK7500 user (I have a Motif XF8), so I can't really speak to your question too much. I did want to say welcome, though.
Universally, I would say, though, it all comes down to comfort level and work flow. It's really hard and ... of limited usefulness to gather opinions from others who try to make music on the internet, as they, of course will have their own biases and agendas in terms of the tools they like to use and why. My best advice is to start with what you know and are comfortable with. Grow slowly and do it for the right reasons. If you find that working with a WK7500 and the home version of Sonar is effective, then there's no reason to change. If you need something more that you don't have, find out what it is, and grow with an eye toward what YOU want, not what others say you should want.
It's an unfortunate trap we all fall into and one point or another... The best music I make (which is comprised of mostly choral type work with me as the sole performer) could be done with one decent mic, a decent interface, and a minimally functional DAW package. What I have is more than I need. I don't regret what I have, as it not only improves my work because I've taken it on slowly and learned how to use it, but it also allows me to pursue other types of music. My point is that I could make perfectly good tracks with much less. The gear is only as good as you are at using it, so take a deliberate and balanced approach when you expand.
Others here do use Sonar, so I'm sure they'll have much more practical advice to offer.
Anyway... welcome again.