Ok I see what you are trying to do.
As long as the HD you buy says 'USB-powered' you won't have any trouble turning it on. It doesn't matter when you turn it on. I have a WD My Passport essential (which is USB Powered) and I regularly boot from it, just plug it into your mac
before you power on your mac and hold the option key.
A bit of advice I'd offer is you can buy a Firewire cable to connect your iBook directly to your MacBook and transfer your old docs, files etc over straight away.
Another way is to download
SuperDuper! (free) on your iBook.
You'll need to have your ext HD for this. Then follow SuperDuper's onscreen instruction to create a bootable clone of your iBook' HD onto your ext HD. (Time Machine is more useful as an incremental back up program, try not to do restores from it.)
Then you will then have your old system on your ext HD which is now bootable.
So when you get your Macbook, boot from the ext HD, then you can create a bootable clone onto your Macbook internal HD. I say 'clone' but I'm guessing you will then use your macbook HD as your main one.
Now you can either keep your ext HD as a backup, or use it as your Time Machine back up etc.
b.