The most versatile thing you can get is a over and under with a 12 gauge shotgun under a 6.5 x 55mm. A well placed shot with right bullet from the 6.5 and it's long bullet will stop anything on Earth. With the right load a 12 Gauge Shotgun isn't a bad backup for almost anything short of the really nasty stuff in Africa.
If I was limited to buying ammo over the counter I would choose from 22 long rifle (well stocked gun stores have it, for a price.), 6.5mm Swedish Mauser or 6.5 x 55mm ,5.56?45mm NATO, .270, 7mm Mauser, 7.62X49 Russian, 7.62x54R, 308 Winchester/ 7.62 NATO, 30-06 and other common calibers that can be found almost anywhere in the world.
The .223 Remington/5.56x45 NATO has loadings that work for one of the best home defense rounds in regards to both over penetration and stopping power and it a capable hunting round for for varmints, deer and hogs with the proper bullet. Next to the 22 Long Rifle the 7.62X49 Russian for the AK 47 is the cheapest ammo out there and some of the ammo it accurate in bolt action rifles.
If you seriously planing to reload I would stick to bolt action rifles to make brass recovery easy and not have to worry about having to make the rifle function. I would look hard at a Weatherby Vanguard in 6.5 Creedmoor, .223 Remington, .25-06 Remington, .270 Winchester, .308 Winchester, .30-06 Springfield or .375 H&H Mag. One of those will work for most anything on Earth. You have a choice of lot more Weatherby Magnum calibers but I wouldn't send a beginning reloader to any of them. The brass is expensive and has a short life.
I would really rather have a rifle chambered for 5.56 x45mm NATO than 223 Remington as the .223 shoots fine in 5.56 chamber but the 5.56 can be over pressure in a .223 chamber.
Good luck
Red
Luck is what happens when preparation meets opportunity. -Seneca