barrel is priority one
most companies make descent barrels for what folks want and will use them for
i went wil E.R. Shaw
now bull barrels don't whip as much and are a little more consistant when repeated fire ... but they are really heavy and at 24" it will feel like your lugging a small child around through the woods
i would go with a 20 and then get the barrel fluted ...saves on weight and gets rid of heat quicker
also i would go with stainless steel, easier to cut and tend to be a little more accurate some say
what Ridge Runner said about the twist is right on
i have a 1X9 twist and had a 1X8 twist and the later would do less then 1/2 MOA easy all day long
now you have to find out what ammo she will like
mine loved black hills blue box in 77gr
75's, 78's 73 grs were ok but not even close to the 77 gr match
good luck and happy building
couple places you can go for parts are rock river, model 1 sales, del-ton and bushy
go to ar15.com to get LOADS of info