Help not sure what to do(child custody)

I have been with my bf for 6 months now and we plan on getting engaged and married and are TTC... Well he has two kids of his own, and their mom made him sign paperwork tht he only gets them one week a summer for her to leave me alone because she has been harassing me but because I can't prove its her I can't do anything.
Well I am getting out of the military and receiving a lump some of money for a disability I endured from the military... 
She won't let him have the kids now for the one week in the summer because he are moving to Arizona and I don't want her having my physical address because she's literally psycho.
Well we talked about using some of my money I'm getting to pay for a lawyer to fight for custody. Well the lawyer he wants is 3500. And I'm only getting about 8300 from th military. And we are using some of that money to start up a nursery and pay some initial fees for college and put some in savings.... Well now idk what to do because paying for the lawyer and whatever other fees come up we won't have any money left over for our child or for savings to keep our future secure.
I feel so selfish.... But I got this money because the military caused severe hip damage and the money is for any medical coverage and ect.
I want his kids to be involved with us as a family and to have a relationship with him but this is just gonna be a on going battle with her...
Idk what to do