I know you said it, but be sure to get an upper complete with the BCG; they aren't interchangeable. She'll need more mags, too, as the 224 uses 6.8mags.
Having a bare upper to swap always seems smart but somehow lowers creep in and make it complete.
Keep an eye out at 208gunshop.com. I picked up several Anderson lowers from them late last year, complete and with MFT grips and Battlelink stocks for $120 each, shipped. It also makes a bit of sense as then you can keep the two different calibers separate so you don't mix up mags and potentially ruin an upper - it happens. I have a 300 BO as well as 223 so go to great lengths to keep them far apart.