Amy,
I have a bit of all the above. Boss Is NICE, but pricy. I have the slighly older Digitech RP250. I think the 255 would be a great way to go! I bought my 10yr old a RP70 last year and he just uses the stock settings, has not tried to programme or tweak any of the factory stuff.
The very cool thing of the RP255 is that he can hook it to his laptop/ desk top and go to the RP website, down load the free X-edit software, and have about 100 different effects at his whim. they also have downloads of 1000's and I do mean 1000's of user made patches you can down load.
http://www.musiciansfriend.com/guitars/ … ects-pedal
here is the info links: http://www.digitech.com/en-US/products/rp255 if you dont want to spend that much look into the 155 or the 70, but the 255 has teh expression pedal.
http://www.musiciansfriend.com/guitars/ … ects-pedal
http://www.musiciansfriend.com/guitars/ … fect-pedal
but for 120 you really cant go wrong. If he ends up like me he will not want to stop there. I love a mix of my processor and stomps. I can set a clean chorused setting on the RP and when I need to boost it for some nastyness in the same song I stomp down on my MXR Full Bore metal.
also to repeat what someone else mentioned Danelectro makes great stuff for the price. I even say for starters the FAB line of pedals is good. I just bought a delay and a chorus for my nephew. at 15 and 20 bucks,, you really cant complain. http://www.musiciansfriend.com/guitar-e … =site1AQBA
When I bought the RP70 for my son, I also bought him a Fab distortion so he could do what I mentioned above.
As far as power,,, all the stomps run on either an AC adaptor or 9V battery. ( suggest for starters, get him a big value pack of 9Volts) On the RP, there is no on off switch, So I have it attached to a short power strip and use its on off button.
Hope this helps.