One of my cats was like this, we rescued her as a stray and I think she had obviously had terrible treatment from humans at some point, she was dumped in our village with another young cat and they both had damaged tails, one was broken and they had obvious trust issues. I've also taken in four ferals that have taken time and patience to tame.
The first thing as everyone has said is to treat the flea problem, then you know she doesn't have irritated skin. I always use advantage or frontline plus, which has worked for me where frontline hasn't.
Toys and playing with her are a good idea, and will help to get rid of any unspent energy that could be causing stress. I find the Flying Frenzy wands and Cats Meow undercover toys are excellent.
She needs to associate you with positive experiences, so offering a dreamie when she approaches you and make stroking just one scratch or head rub, no more.
There is a school of thought that says that cats can experience electric shock like sensations when they are stroked, which can account for the sudden attacks, and also they can get over stimulated making them go into attack mode. I'd really recommend watching some of Jackson Galaxy's videos on Youtube; they have some excellent tips on dealing with aggressive stressed cats.
She also needs her own safe places that she can retreat to and not be disturbed. If she's scratching furniture, and to be honest most cats will, then it can be deterred with using double sided sticky tape.
The main thing is to let her come to you and don't overdo the fussing, I think that they can settle more with age too, the cat we took in that was like this turned a corner when she was about six, we'd had her since she was about a year old, so it took five years of building trust, but she will come and be happily stroked, we know when to stop as she will gently bite, but it isn't the full on grab and kill thing that she used to do. I also had an old grumpy cat that was always like this, it was just how he was, but he also mellowed with age, we still all loved him and missed him when he died.
I'd give her time with trying to accomodate her needs more for now, but if you still feel you can't live with her in time then please use a reputable shelter to rehome.
I don't think living as a farm cat would be fair for her though when she has always been domsticated.