In short: yes, Renoise, yes yes yes, and don’t worry about “too complicated”, that’s just the self defense of us slow, obsolete adults, let’s face it Seriously though, you can’t go wrong with the demo, can you? A computer suited for Renoise is also going to be usable for other audio apps, so I wouldn’t worry about that too much, it’s not like the kid isn’t hopefully going to try out gazillions of audio apps anyway
I have no specific hardware advice, just general thoughts. Firewire and plenty USB ports is good, RAM is good, not having a crazy loud fan is good, onboard sound tends to be crap but maybe you can find a laptop with onboard sound that rocks (?) which would be neat, although for super-serious recording etc. you prolly’d end up with an external sound card anyway (?)
Renoise is definately “good” enough. Regarding “easy”: there are many programs that are easier, but also vastly more limited! The Renoise GUI is great, there’s tooltips everywhere and configurable shortcuts for everything, and then there’s the documentation, too. You can’t get much easier than that without limiting potential.
What’s important is that you can get first results quick (and that’s the case with Renoise, at least in combination with a short tutorial), because having to read and experiment for weeks before you get some minor gratification is no fun.
What’s also important is that there are many many songs that can be inspected and remixed. I know this is true for Renoise. I’m not sure how common it is outside of tracking (this would interest me… do for example Fruity Loops users tend to share their creations?).
And then there’s this huge legacy of awe inspiring .mod’s and .xm’s, if even just 10% of the most kickass tunes load well into Renoise that would be a lot to dissect and learn from.