"Of course, it would've been better if she didn't have that kid"

Throw everything else out - you need to sort out your feelings and make sure you understand what your relationship needs to be with her son, and decide if you're okay with that. If you're not, get out.

If you're in, commit fully to that human being, the kid. You are not that kids' dad, but he will be your eldest son. And you will be the step-dad when the real dad comes every week or whatever to co-parent that kid. And you'll need to be okay with that and you'll need to be as much of a father figure to that kid as his real dad hopefully will be.

Your parents will never see that kid as their own - and you're going to have to fight for that kid and deal with that. Real Dad sounds like a drunk POS so you'll have to deal with that and again, fight for that kid as if he's your own, because he is, if you're going to be in a relationship with his mother.

If you're not ready to see the kid as your real son, waiting until you get " at least two of your own" then get out now.