I had PND with DS1, and am currently 31wks with DS2, and I had about a month early on where I was sure the PND was back with a vengeance - about your stage now IIRC.
I saw my GP, and we decided to hold off meds and see how I got on - the 'PND' lifted, and I've felt fine since about 16/18wks. I don't know what it was, but it was horrid and affect my thinking on everything, so please try to think about what will make you happy long term rather than what you think will fix how you feel right now.
I absolutely support your choice to abort, just want to warn you that this may not be PND so please don't assume it is.
Seeing as you were initially happy with the pregnancy, and weren't sure your family was complete, you may benefit from a talk with a counselor as well as/rather than someone emotionally invested in the pregnancy - after all, a third DC will have the same impact however long you wait (HG, disruption to DCs 1&2, SAHM etc), so really the decision is whether you want a DC3 at all.
If you do choose to go ahead with the pregnancy, I think a nanny is a brilliant idea, try to leave your preconceptions at the door, you can tailor the position to suit your needs. It would allow you to have some freedom from home, and for life for DC1&2 to carry on more normally. When DC3 weaned you could then go back to work at your own speed.
Also, if PND did hit again, you would have home support already in place.
Everyone always seems to say "Once they were here we didn't regret keeping them", but I know I don't want a DC3, and would be in your exact shoes if we had an "accident". Heck, I'm already crapping my pants about having DC2!
You're under a lot of pressure time-wise, I hope you can make some calls and speak to someone at Marie Stopes or similar today for some help.