Stop crying, get a grip etc... ect.... 
Me....Nearly 41, 3 dc's 11, 9 and 8, Dh 36.
Got "surprise"! pregnant last year. Found out shortly after DH lost his job.
Who cares what anyone else thinks?! It's what you as a family think that matters.
We are, possibly in a slightly better position, as we already have a 7 seater, but if you trade in your old car you might get a good deal.
Sleeping arrangements are another matter, 3 bedroom house, no room for an extension, and no chance of moving, however we do have a cellar which only needs a little bit of tarting up, and that will become bedroom 4.
If you are newly pregnant, then you have a year or more before you have to worry about a bedroom for the baby anyway, and plenty of time for any alterations.
Try and think practically. Another baby will mean an increase in tax credits and child benefit. If you give up work then you might get more, but many people keep working no matter how many children they produce. Older children can help around the home and with the baby. (Or mine will anyway, when it arrives)
I bet your other kids will be over the moon about a new little brother or sister. Congratulations!