The Vanquish should work, but I'm not sure she needs long-range firepower. She's great in close quarters, but I doubt she has any long-range skill. Also, she should still have Marissa's Walther PP (.32 ACP) with suppressor and shoulder holster. I'd get her a Luger P08 as well, she had one in Finland and is might be nice to have one now. Same with a bow, she knows how to use one and going Robin Hood on bad guys is cool.
Slightly off-topic, but does she have friends in the fanfic? Aside from the bonus of having a second person to watch your back, she's still a naive teen with very little idea how the world works. She could seriously use a big-sister sidekick. I'd give her 2 buddies. Ass-kicking big sister type to cover her six in combat (two is always better than one) and teach her how the world-and, more importantly,
boys
- work. And a slightly nerdish type (male or female) to act as driver (fast getaways are nice) and be the adult of the group.
That said, they should
try to follow the law. Try to avoid full-auto, and put all the suppressors and any SBR/SBS in an NFA trust. AR pistols could work as a substitute for assault rifles sometimes.
Yes, I just gave her 3 more guns, a bow, and 2 more people to give guns
. But by herself, she can't do too much and remember, she can't even get a hotel room, too young.