GUNSRifle:
A .22 rifle is useful for small game and portable; a .450 Marlin rifle can bring down a South Dakota bison, . . . but is cumbersome, especially for a 19-year-old girl.
And in any case, for traveling light one must make compromises.
DonDiego will recommend an unlikely compromise: the
Marlin Camp Carbine in 9mm. This rifle was manufactured from 1985 through 1999 and is readily available used. DonDiego supposes the ammunition stores will no longer be open post-apocalypse; the 9mm cartridge is presently the most popular handgun cartridge on the planet so it should be among the more available when ammo's no longer being made.

Handgun:
For close-up defense one
needs a revolver with stopping power; as a minimum DonDiego would recommend a 9mm, perhaps the Ruger LCR. (In general one cannot go wrong with a Ruger; they are tough and "accurate-enough". And Made in America.)
This would also allow the heroine to need only one cartridge for both her handgun and rifle, a significant advantage.
However, for
real stopping power DonDiego suggests the
Ruger Single-Six in .357Magnum:
n.b.:
__A .357Magnum handgun fires the less powerful .38 cartridge as well as the .357Magnum, so it provides some versatility. One needn't employ the larger caliber when it is not needed. However, the need to carry several different calibers may be impractical.
__DonDiego recommends a revolver over a semi-automatic in any case because of reliability and simplicity.
Shotgun:
If one is afoot it may be unwieldily to carry a rifle, a handgun, and a shotgun. But if one can, just carry a 12 gauge
Remington Model 870. It is the best selling shotgun of all time for a reason.
OTHER PRACTICAL ITEMSA tactical tomahawk is a useful tool as well as a potential weapon. DonDiego doesn't know enough to recommend one; and the heroine pr'bly cannot be very picky anyway.

An axe.

A Hammer.

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ABSOLUTE NECESSITYSPAM. The heroine should seek SPAM at every opportunity.
NICE TO HAVEAt some point a nubile 19-year-old female will experience certain urges, . . . the descriptions and resolutions of which DonDiego will leave to the singular writing skills of malibber2.
Subsequently, the young lady may have use for the War Stroller.

DonDiego awaits the completion of malibber2's coming tome on the apocalypse. And a complementary signed copy, . . . hopefully with the author's signature and a mention of poor old DonDiego in the acknowledgements.