toots I am so very sorry to hear of the condition that is affecting your daughter.
I have no idea if you would be elligable to adopt but if you wish to find out you must speak to someone in your county couoncil or local authority adoption services of a voluntary agency.
No one here will be able to garantee a 'result' either way but we can certanly talk around some areas of the subject.
If you dd is at school part or full time then you will have some time to devote to a new child but eventually both will be at school and the time to be 'shared' between the kids will be the after school and weekend time.
The points made by others about a child who has suffered loss and trauma entering a home where there are significant needs and possible loss of a sibling are not things you can get away from, but I do think it would be good to discuss it with professionals. It would also be good for you know and be able to say how you would prioritise the needs of both children.
In your shoes, if money were available I would want to consider either the use of a surrogate or fertility treatment with donor sperm and egg or donor embyo. If you will need donor sperm and egg (as your post suggest the medical condition could come from either of you) then I would say treatment with donor embryo will be much cheaper. Your best bet is to consider treatment in a clinic in the UK and also to look at clinics overseas. I know people who have tried treatment in the USA and Greece and when we considered this we were looking at Spain. I am happy to talk more if you wish to pm me or ask me here.
Just for the record we have a 10 year old birth dd and we had treatment with donor eggs to try and have another child, (sadly it did not work for me - probably because of immunology issues with my body).
I am VERY happy that now we have a son who joined our family by adoption. We do have quite a demanding dd and the relationship between the two of them is sometimes great but at other times hard.
If you are healthy in terms of carrying a pregnancy you may be able to carry a child created by donor egg or donor sperm or donor embryo and would be able to fit that child around your existing situation more easily than an adopted child (IMVHO).