First of all, make sure you have insurance before you get pregnant. Pregnancy can be a wonderful experience, but the last thing you need is the stress of worrying about money.
Second, worrying about whether you'll be a good mom means you are concerned enough to want to do your best. Nobody really knows what kind of mom they will be until they have their own kids. If women decided not to have children b/c they were unsure of what kind of mom they'd be, we'd live in a very underpopulated world. Fortunately, you are gaining experience as a mom right now by having stepchildren. Yes, I know you are not their birth mom, but you are still a nuturer and provider to them.
Your fear of the baby not being healthy is a common concern. However, having a baby is kind of like playing Russian Roulette. Fortunately, the odds are in your favor of having a healthy baby without complications. Most babies born today are perfectly fine, so don't spend too much time focusing on the negative things that can happen. Try to be positive.
Try sitting down with your hubby and explain your concerns to him. If you are not mentally ready to have kids he has to try to understand that. He has two wonderful children, so maybe he can focus more on them rather than some child who has yet to be concieved. If things escalate to the point where you feel the topic of getting pregnant is causing real problems in your relationship with your hubby, then seek counseling.
I hope this helps. BTW, I am a SAHM mom to 2 beautiful girls, ages 2 and 6.