I am so sorry for your ordeal, make sure you keep elvis, and your wife as she has had the shock of her life, on rescue remedy for several weeks.
When elvis is recovered, and don't worry they adapt a lot easier then we do, you must address the psychological damage. Elvis needs to be re socialized with dogs and especially so with staffies as he might get very fearful of them; a fearful dog is a target for another dog, not just staffies. Fearful dogs are unpredictable and give out wrong messages and get attacked to prevent an attack. Fact! My dog was attacked and for weeks after he kept being attacked on a regular basis by all sorts of dogs, even the ones he was friend with.

My dog is a staffy cross by the way, he has fear issues which we work on, he has been attacked today by a boxer, 3 times by labs, and nipped countless time by small dogs. If he can avoid confrontation he will, but if cornered like today, he was under a table on leash he will retaliate.

Most insurances will have third party cover, that would pay for the whole of your vet fees.
By the sound of the way these two dogs were kept the owners probably never bothered with insurance and if they fit the picture in my mind they probbaly are on some sort of benefit and get freeish vet from the rspca, blue cross, etc. One of the reasons so many idiots own and damage dogs, which ultimately pay the price for their owners' neglect.

If you find yourself in a similar situaton in the future, rather then avoid the dogs you can try to have them all walking on leash together side to side (without the dogs being next to eachother), that might relieve a lot of the stress dogs feel when they meet eachother.

Try not to fear every staffy you find in the future as dogs feel energy and I know it's hard, but I see how everyg=body jumped to the opportunity to curse the breed and ultimately it doesnt do any good to dogs living among dogs. I take ray to socialization classes and out of the 20 dogs there mostly were muzzled except the staffies, amazing uh!

Watch the handler more then the dog, and often even though if you are fearful try not to show it.

I wish you all to make a full recovery, I was shaken up for months after ray got bitten and he suffered the price for it. They recover a lot faster then we do.