Primary Arms 1-6 SLX ACSS LVPO 5.56 FFP: Comes with Aero Ultralight mount and throw lever. Used for about a year. $200
Primary Arms 1-6 SLX ACSS LVPO 7.62 FFP: Comes with Aero Ultralight mount and throw lever. Never used. Bought for a 300 BO upper that never was completed. $200
Athlon Cronus...
Also AMD Ryzen processors are more efficient and better performing per watt than their Intel counterparts. That would help extend battery life further if you are opting for the Windows route.
If shes ok with Macs and software is not only for Windows. A Macbook Air 15 inch is good with long battery life. Apple Silicon is the best for being away from the plug. You can check out Best Buy open box returns to lower the purchase price even more.