I first want to say that I'm deeply sorry that you're going through this. It's not something anyone should have to deal with, and I absolutely understand you wanting - and needing - to do whatever possible to protect your family. I'm wishing you lots of luck in getting through this unscathed and returning to a place where you and your family can start to recover.
Onto the guard dog side of things. A genuine protection dog - one trained to actually bite an intruder/target - is legally considered a weapon. The training really needs to be done by a professional and is very intensive, similar to police or military training, and yes, requires a dog with an appropriate temperament. This is because protection training done poorly creates a dangerous dog, one who may decide who to bite on its own without being directed to, or who starts biting out of fear or frustration. I also believe you need special insurance to own a protection trained dog.
I'm basing some of this on your other posts, but here are my suggestions:
Invest in cameras. They're getting more affordable all the time, and you wouldn't need a very high-end one to be able to see if anyone's hanging around your door that you're not expecting/wanting. Might also be very useful if you do need to pursue legal action, or even deciding whether you should (I know restraining orders don't save lives, but just covering the bases). At the very least, they'll be able to offer you peace of mind.
Train her not to guard, exactly, but to alert. Teach her to bark on cue, then teach her to do so when things happen like the doorknob rattles, door opens, etc. Or you could get creative and have her ring a bell or horn or similar, so it's more clear that she's alerting to the door rather than, say, a cat out the window. I know this is the opposite of what most dog owners want, but your situation is exceptional.
Reconsider the invisible fence. They have downsides to begin with, but if you suspect she's been abused with the collar in the past, you have an extra consideration where she may become fearful or reactive when the collar's on and/or near the boundary of the fence due to her previous bad experiences. I hate to even bring this up, but an invisible fence also offers no protection if she's outside alone and someone wants to come onto the property to hurt her.
Give her time, and start looking for resources for fearful dogs, since she's now uncomfortable around strange men. Don't force interactions when she's giving you signs she's not okay, and don't punish the growling - it's communication that she's scared or uncertain. Dr. Patricia McConnell is an experience animal behaviorist and has an inexpensive book called The Cautious Canine if you want a place to start. I have not read that one specifically, but have other books for different issues and her writing is very clear, digestible, and solution-focused. There's also a lot of resources online, but I'd urge you to focus on ones that use force-free methods - fearful dogs and training that uses physical force or intimidation don't tend to mesh well.
Again, I wish you so much luck and hope this is only a brief chapter in your life. You did an incredibly hard thing by getting out of that situation, and I hope something I suggested will help in some way.