It is a hard choice. I guess you need to think, would you ever regret not having a 3rd child in the future?
My only thought is with your history it could be a bit hard on you if your still TTC when things with your dad get more to the later stages. I could be a lot of stress for you and emotions will be very high.
But you never know, maybe it will be different this time. With regard to will you manage. We all find a way and you will too. And a new car is always a nice thing and what kids don't love bunk beds if that is what has to happen.

Goodluck with you choice and I am so sorry to hear about your dad.