I am another SONAR user that is happy with the Focusrite brand.
I have a couple-year old Scarlett 18i6 interface and it has proven to do everything I and SONAR demand of it, easily, with power to spare.
I ain't gonna tell you any other brand or model is crap, but I will tell you that I've been dabbling in this electronic music making/production thing since I got my first Twelve Tone Systems Cakewalk Pro software sequencer back in the very early 1990s - and my Focusrite is just as fine an interface as any I've ever used over the years.
Just make sure: If you ever want to use a microphone (and it sounds like your daughter will) then it will be best to decide up front if it is dynamic or a condensor model that you want/can afford. If it is a condensor model, your interface (whatever brand/model) will have to have 48v phantom power on at least 1 input for that mic. Dynamic mics do not require phantom power.
Also make sure that the interface that you choose will connect to your laptop - if you buy one with either a FireWire or USB interface, your computer will need to have a FireWire or USB interface to plug it into. (I've seen folks complain online that they didn't know their computer didn't have FireWire *after* they bought a very pricey FireWire-only interface).
As to external drives: Get what you can afford. USB works fine. eSATA works fine. FireWire works fine. Unless your daughter starts doing 96 track film score projects (in other words, HUGE project & audio files), the transfer rates of the various HD interfaces will be difficult to perceive with smaller projects. I personally use several-year-old plain old USB external MyBooks from Western Digital and they work just fine. I just used one to download my X3 upgrade to (multiple GBs), from my desktop computer, before copying it to my laptop computer for its unpack and install.