Hi there vivitan.
Personally I feel that this has to be a joint decision between you and your hubby and of course your two daughters. You know your situation best.
If you were to ask me - personally I think your daughters will be ok with a sibling. ESP since your eldest is 16 and younger is 5. And I trust there are no hard "step parents" feeling.
From what you are saying, it also feels like you and hubby want a child of your own but yet it also feels like you are not sure cuz you've cited so many factors on why you should diss this idea. If you really want something, you would make this work.
out of the factors you've cited - docs, health, finance, care, age... I feel the most impt is age. at 40 - more tests and higher risk - are you prepared to accept the consequences if the tests are not ideal? Also, are you prepared to continue working until beyond retirement at 60 as you would need to support the youngest child? That said - I have had friends who went on to have healthy kids at age 40, 42, even 45. But they recognize that they probably won't be able to enjoy retirement years until much later.
Health - I am sure you will nurse yourself back to good health if you are preggers. Docs - many docs in Singapore - you can do a review and pick and choose. Caregivers- infant care and nannies are options. Finance - yes a valid concern but again - look at how low income families have had many kids but still made things works - it's a compromise.
I am a ftm. So maybe I am being too naive. =)
Whatever your decision is - make sure it is a family decision. And yes always consult your kids. dont't leave them out.