I felt the same way when I found out I was having twins. In fact, I'm still struggling with the fact that I now have twins, even though they are here (I just talked about this in my post - Twins as 3rd and 4th Children). No judgment here. How are you doing? Nanasi had some really great bullet-points, and everyone else has offered great advice as well. If you decide to have the twins, you will somehow figure out what you need to do to make it work. Will you enjoy it? Who's to say, but that's why we have antidepressants (spoken somewhat tongue-in-cheek - I'm exploring natural remedies). As everyone else has said, the first few months will be tough, but I'm sure that in at least 95% of all cases, the sleep deprivation is alleviated eventually...Yes, twins are a lot more work than 1; yes,they will both scream at once and it will be unsettling, but I'm guessing that you will physically be able to handle it, and most people seem to say it gets easier at 4-6 months.
I was thinking about your apartment situation and remembered that one of my twin parenting books talks about this. It suggests carrying one in a carrier and the other in one arm, leaving another arm free to hold onto the railing.
It also suggests putting them in a light-weight side-by-side stroller and backing them down/up the steps -- proceed with caution! Once they can hold up their heads and are less wobbly, you might be able to carry them both at once fairly easily? If none of this sounds workable, what about sub-letting for a bit?
As far as having to stop working at home - could you hire a student as a cheaper "mother's helper" who could entertain the kids while you work from home? Also check out colleges with nanny programs -- I'm sure there are quite a few in NYC? I know sometimes the students have to do a practicum in a person's house as part of their degree -- you could sign up to do that, and it shouldn't cost you anything. You would probably have to be at home while the student is there, but you are working from home anyway. Could you make more $$ if you WOH part-time, and therefore be able to afford chld care for a few days a week? Personally, I find work to be a refreshing break; if you feel the same, I would seek out any way to continue.
Finally, regarding the relationship with your dd who will be 22 months, and her potentially feeling left out b/c she is not a twin: IME, this is surmountable. They will all be close enough in age that they will likely all play together at some point. Also, it's likely that at some point, the twins will be sleeping at the same time (morning nap, afternoon nap), and your older dd will not be taking a morning nap, and then eventually not taking an afternoon nap, leaving you a lot of time to spend with her individually. If this doesn't happen, you can always carve out some time for your older dd individually and leave the twins with your DH. If there's one thing I've learned about having four kids (though I'm still new at it!), it's that the kids like that there are now more kids. My 5 year-old and 2 year-old love to interact with each other, and they both think it's pretty neat that there are 2 babies on the scene. Who's to say whether they will someday be on a therapist's couch complaining that they were displaced by twins at a young age -- right now they same to be enjoying it...
Obviously no one else here can know whether this is right for you, but I do think most of us humans have an innate ability to adapt to very difficult situations and to come up with workable solutions. I have no idea whether you (or any of us) were "meant to do this," but I do think its doable if you do choose to do it. Again, it may not be something you're happy about right away (I'm hoping my outlook turns around eventually), but you never know how you'll feel about it when they are 6 months, 1 year, 5 years, 10 years, and so on.