Oh very 
Foxes are a huge threat to guinea-pigs they are relentless ( I won't use the vermin word , I've been told off before )
But they are brave and have cubs to feed. My NDN got used to me roaring down the garden clanging metal plates together to scare them 
Badgers can be predators but less likely .
Your guinea-pig will be in shock yes they are very vunerable little prey animals . Keep her indoors for now to keep an eye out .
Regarding cages etc there's lots of advice on rabbit and chicken sites for protection. The Eglu run was originally designed to keep chickens safe then branched out to rabbit/piggie . They're quite small inside (and ££) but easy to clean, keep and optimum temperature and you can get security items like a 'digging skirt' ( extra mesh round the perimeter)
The locks lift&twist .
I put a padocl on our guinea pigshed and a bucket full of cement (weighed a ton) in front of the door . It was on a solid base too.
With our runs we pegged them with heavy duty tent pegs (they have a steel peg at each corner but it wasn't enough IMO )
If you do offer her back to rescue she'll have the company of other guinea-pigs . The Rescue I got GP7/GP8 from had a big shed of sows in groups .
Or if you want a cagemate I can recommend a lardy middle aged neutered boar . There will be boars that have been boar pairs that had to split up . Or lost a sow .
Boar/sow pairings work well - personally I found it easier than sows .
Some rescues will do a loan of a piggie to bond which might work for you (but you;ll fall in love with it )
Give her lots of cuddles and see how she goes but ,yes,prepare yourself 