DH - I suffered from PND for 18mths at least after ds was born!
When he was 2yrs old I felt ready for another, although had exactly the same fears as you have (although i didn't have the scary birth experience you had).
I got pg a bit earlier than planned when ds was 2.3yrs old and spent most of the pregnancy in total terror at the prospect of hitting rock bottom again and not being able to remember the 1st year if my 2nd babies life either!
My PND was undiagnosed until ds was 18mths old, I felt better and went to the GP. By that point I was over the worst and didn't require any medication.
I spoke at length with my GP and one of the MW's about how I was terrified of PND again. They assured me that this time it would be spotted immediately, and as I was being honest and open about how I'd hidden it from everyone I wouldn't slip through the net again. That made me feel much better. At 32 wks pg I slumped to a low and the MW recommended doing the Postnatal Edinburgh Test where I scored 18 and was prescribed AD's. I didn't take them, but felt better again as I realised that if anything turned 'bad' after the baby was born I'd be taken seriously and not left alone to cope again.
Dd was born at 39 wks and I sat waiting for the PND to strike me down...... I waited and waited. When ds was 9 days old I can vividly remember that being my last day of being happy.... so when dd hit 9 days old without consequence I was so happy. Everything seemed easier, I couldn't believe how proud and happy I was with dd.....feelings I am ashamed to admit I just didn't feel with ds.
But now, 8mths on, I have not suffered from PND. Despite having some rough times, and going through a bad patch with DH I don't and haven't felt the deep sense of hopelessness that I felt back then.
My HV came around every week and spoke to me about my feelings, and taught me relaxation techniques to cope with anxiety and she really spent time with me which helped so much.
Just be honest with your health carers and push for support if it isn't offered (which I'm sure it will be).
I don't know if it was the mental preparation and support from the GP, HV and MW's that prevented PND striking again, or if I was just not unfortunate enough to suffer twice. Doesn't matter. I prepared myself for the worst, but kept up my optimism by knowing that if I did suffer again I would have lots of support and understanding.
When you have another baby, the reality is you are more likely to suffer PND as you have suffered previously. But just be prepared and realise that this time you will seek the help that would have perhaps got you through it quicker and easier.
Good luck! Go for it! Please don't live in fear of PND, you really can gain some control over it and fight it head on before it gets hold.