You can easily have velocity control the volume :) or make it control the amp env sustain, which is one of the techniques used when composing SID tunes on a C64. In fact, velocity can be used to control almost any parameter.. vibrato, PWM or whatever, as Midibox SID v2 will treat velocity data just like any other modulation source
And now, back on topic :D If you want to make the absolute best use of your keyboard, then make sure you get one that handles sysex and NRPN, because the CC scheme only works for the lead engine. Now, if you get a keyboard that supports NRPN, then you can easily control ANY parameter in your SammichSID in ANY of the engines :) Over at ucapps.de you can find the manual for Midibox SID v2, and in the back you will find looong lists of all the synth parameters and their respective NRPN numbers. You then assign these NRPN numbers to the controls on your keyboard, and voila :) you're rocking!
Sysex are of less importance, but allows you to do a lot of other cool things. Most important are the NRPNs if you want to control Drum engine, Multi engine or Bassline engine besides the Lead engine. Lead engine is the only one also supporting CC numbers, so if your keyboard only supports CCs, then all you can control will be Lead engine.