EggXpert

So, my fiance wants to buy a MacBook, but I'm not sure which to get. All she really wants it for is Garage Band and music stuff. Sure, she could get an iMac, but she wants a new laptop. I'll probably use it for iMovie too, but I don't know. Will she be good with just the basic MacBook with no upgrades? I think I might have her get Office for Mac, as well, but I know that doesn't take up a lot.

Laptops are pretty good at that kind of stuff, you should be fine with any basic laptop for what you discribed.

There's a relationship between processing power, battery life and heat.

Generally, the faster the computer the shorter the battery life and the hotter the laptop gets. That would really be the only thing I would warn you about.

I'm not up on Macs but I haven't heard many complains from users, however I attribute most of this to the fact that Mac users are generally not gamers and not power users but more artists and web surfers.

If you do go over basic model, spend it on a larger hard drive and more memory....not graphics or bells and whistles.

for office/editing type software, the $400 laptop is on par with the $1700 one.

Well, we're not gamers but I definitely would not say we're not power users.  The creative professional applications can be very labor intensive.  All the programs and activities you've mentioned though, should be fine for a regular Macbook.  If things are a bit slow, it wouldn't cost much to add some RAM and speed things up a bit.  If you have an Apple store near you, I would go there and check one out.  Everything you would use the Mackbook for will be on the display models and you can see how well everything works.