1.25-28V var. power supply mod
You can get a simple LCD panel voltmeter from Futurlec, they have various models. The PM128E for $7.90 looks like it would do what you want. I'd suggest using this instead of trying to use the brightness of a bulb to show the output.

For your fan, probably you should just have a separate voltage regulator to supply power to it, since it needs a constant voltage, and not an adjustable one.