Hi, just thought I'd let you know we are in a very similar situation at the moment. My husband has IGA Nephropathy which is an ongoing kidney disease, he has just had a check up with his consultant and discussed our plans with him - he said he could give a report that would indicate the prognosis to be reasonably good but our problem would be protein in the urine as my husbands is a constant +3 and apparently they don't like this. I have just emailed ASA who have given us a free assesment and we get 3 free questions, I am still waiting for a reply but it may be worth you contacting them to see what they say.
We are looking at the possibility of paying for my husband to have a medical before we apply for the visa just so we know we aren't wasting about £4k+ This will probably mean paying for two medicals and obviously there is no guarantee but it would give us a little bit of piece of mind.
Also I would like to say thanks to the informative posts on this thread