My Dad's sister has Downs syndrome. She has quite severe language problems but thats pretty much due to the fact that the doctors told my grandma not to bother cos she'd never talk anyway. My grandma had postnatal depression and no support, back 45 years ago, she was advised to give her baby away to a long stay hospital to be brought up in an institution. My grandad convinced her not to do it, but my aunt spent a lot of her early years in a pram without anyone talking to her and no toys. I think if you did that to any child they would have problems!
Now she is a very loved and full part of our family. She comes and stays with us for chiristmas, weekends etc, she'l phone my dad and say "coming to your house?" if she wants to visit. Socially she knows how to behave really appropriately. She cant read cos she was never taught to, but when we eat out she will sit and look at the menu, to any onlooker you'd think she was deciding what to have, then she leans over the whoevers near her and asks what there is and decides. Then she can order herself, (when the waiters give her the chance...but thats whole other issue!)
She lives in supported living, she doesnt cook for herself but she does cooking in college at the mo, and Im fairly certain shes capable but the carers in their house do the cooking for health and safety reasons :/ she helps out with dinner at ours tho.
So I suppose, while my dad would be considered her main 'carer' she's very independant. A lot of her dependance is learned from being overly cared for. For example, she will not get up in the morning when she wakes up, she waits to be told to get out of bed. We've tried everything but thats how its always been for her. Its hard to know if she actually wants to get up or wants a lie in. Sometimes when I dont get her til 9 I worry that shes been awake since 7 waiting. Also her language problems are unusually bad for an adult with downs syndrome, most of the girls in her houseshare have jobs in local shops etc... and are really sociable. but my aunt is never front of house cos she just cant communicate well enough. She needs an awful lot of guidance with fashion tho
my sister and I usually tell her to change her top cos shes combines some strange clothes, she loves girly time and shopping, doing her nails, she loves it when we do her makeup too, something she was never allowed for some reason.
Hope this helps. Ive just blabbered on really. If you did have a child with Downs syndrome, you would have a huge amount of support to help your babys development in the early years. Most children with Downs syndrome who I work with go to mainstream high schools and take some GCSEs. Theres no reason why someone with downs syndrome should be illiterate unless theyre just not taught or if they have more severe learning difficulties/ dyslexia. Yes your daughter probably would end up being a main 'carer', but I dont believe it would be a massive burden. More just going with her to hospital appointments, and supporting financial decisions etc...