Man I think the 7saum is a great choice but maybe not the best for your lady.
I got a 270 wsm for my wife a few years ago thinking I could load 130s for lighter recoil.
I ended up making her recoil shy and she developed a serious flinch, caused her to miss a bull at 50 yards.
She has a 7-08 now and got over the flinch.
My advice is to seriously consider your objectives before getting her a magnum.
For what it's worth my buddy just shot an elk at 450 yards with his 243 and killed it instantly.
I shot two bulls this year with my lightweight 6.5 RSAUM and they too dropped in their tracks (Very light recoil btw).
Point being you don't need a hammer.
As for barrel length, I would go with 24" to reduce muzzle blast, that alone could make your wife develop a flinch.
Good luck