You would bond with the baby (I don't doubt that - if I did I wouldn't say it).
I'm a great believer in gut reactions so if your gut is that you wouldn't do it then take comfort from going with your gut. I think it is important to imagine how you would feel if you did get to know a child with DS in the future (I don't know a nephew born with DS, or a neighbour moving in or something). Because its a decision that you have to live with I think itsimportant to know that you would still feel that you made the right decsion in 20 years. It's kind of putting yourself in all the potentially difficult positions. Even then though your reaction may differ depending on the particular child.
If you did go ahead and have a child with DS- what do you know about the future? Well that's very hard to predict because DS is a spectrum. At the high functioning end you have a child who would attend mainstream school (and at the very high functioning take GCSEs, although that is probably very untypical), and at the lower functioning end they would be in an SLD/PMLD school and possibly never talk, and may well have challenging behaviouirs. You mentioned possible health problems- I think those need to be considered as well, as tey do add an extra dimension. Again Eidsvold has been through that with her dd.
How would it affect your other children? Well most research on siblings show that they grow up to be maturer and very well adjusted. Having said that life is definitely harder for a sibling of a child with SN, and they do miss out on some material things (easy holidays,family trips to cinemas restaurants etc can be harder- although I;m a great believer now in just doing thigs individually). Just by coincidence in this weeks Daiy Mail (eek! don't usually read- needed the financail mail) there is a piece in the magazine by Jo Whilley who's siter has cri du chat syndrome. I thought the piece was very honest, and her childhood appears to be very similar to ds2's. I hope ds2 and ds3 grow up to talk as positively about their childhood as she does (she had the same constraints about going abroad etc). She said that heer sister has been a very positive influence on her life- but she did test for cri-du chat and said she would have terminated iof she had been carrying it (no idea at what stage she tested though-I think that alters the ease of making a decision),
I know very little about having a late termination, but I think it would be important to try and find out as much as possible about it before hand. Even if only to see if there ways it can be made easier or things that can be done afterwards to help wth grieving etc.
As regards people on here who already have a child with significant disabilites, some would choose to terminate in future pregnancies, some would not, so there isn't any set way that coming into contact with disability makes you feel.
TBH I think you'll feel guilt whichever decision you make, so I would bin that right away. Hopefully the tests will come back fine, but if not just remember that you have been put in an impossible situation, and you just have to make the decision that you can most easily live with.