I haven't built the Zen, so I can't comment on how it sounds. My best guess is that it would sound a bit like a tube Class A amp, e.g. a Fender Princeton or comparable.
You could certainly power the preamp stage from the Zen's power supply. Most commonly available JFETs will only handle about 30 volts. A simple 3-terminal regulator or even a Zener diode can do the job, the current involved is not very large, maybe a few mA.
I personally would not want a fan anywhere near my guitar amp, especially if I wanted to record the output! Use a big honkin' heat sink from your local electronics surplus store.
If you intend this amp to work at "bedroom" volumes, you really only need a watt or two. Driven into power amp distortion, 10 watts with most guitar speakers can get very loud! Start small and scale up if you need more volume.