You need to know how high, you can't just use the light above the canopy. You need distance at least a foot between canopy and the light.
I say 4 plants. Depends on strain. If autoflower 8-12 maybe? I'm not familiar with those.
If you use an effective exhaust in the cabinet yes passive intake from the door can be enough.

Go with leds if you worry about the bill.
If anyone tracking your electricity usage (per hour, per day) yes it is possible you can look suspicious.