It depends on what you want to record and in what quantity at the same time. And how often you intend doing it
Questions like:
Just Audio recording?
Or just Midi recording?
Both?
Both simultaneously?
One instrument at a time ?
More than one instrument at a time?
Will you use soft synths?
Will you use plug in fx?
16 bit or 24 bit record and play back?
44k1, 48k or 96k bit rate recording and playback?
Once a day? All day? Once a week? Fortnight? Month?
What is your future goal? Own studio? Never more than a bit of your bedroom/loft/basement/garage?
And whats your budget? For now? Next year? Eventually?
Because you have a mixer you can sub-mix and just go into your sequencer software with a stereo pair of inputs. With Sonar and all the other sequencers, its not necessary to have a mixer, just more inputs on your soundcard.
Sonar is
a ProAm software tool that can be used by Home Recorders to get some great results. But Cakewalk caters for others too. For guitarists there is Guitar Tracks and for the less professional/students of the medium of recording, Cakewalk offer a cut down version of Sonar called Home Studio 2002, as posted by my friend brzillian

You can always update later for less money coz Cakewalk have a great upgrade policy
Hope this helps.