Yeah, Supercollider would probably make a good engine. Could probably whump up a fairly good, cheap delta-sigma DAC output for it.
The old Synclav II was quite the DAW for its time: sample and wave table engine, FM engine, sequencer, interactive wave table editing, light pen monitor, a quite revolutionary music notation function, and so forth. Dual 6809s (w00t! w00t!). The most expensive parts would have been the electronics - the thing cost roughly $20,000 in 1980s dollars.
Nowadays, the most expensive part would be a weighted-key velocity-sensitive keyboard. For cost-effectiveness, a MIDI keyboard to soft-synths on a standard desktop or laptop is probably the way to go, but a dedicated DAW is still more convenient as an instrument - less setup and tear-down.
