You are more important than the company. And your future colleagues, will agree with this, I think. Companies are built to make people happy. Not the other way around.
(I think I a little bit can feel how you feel about joining the company and then later on they will know that you have this illness. And I think that they will think you did the right thing, when you joined. Assuming you don't over work yourself, but take time to rest when you feel you need to.)
Think about your parents and your fiancé. Those who are close to you, matters more than [a relatively small amount of money to your future colleagues. Or a large amount.]. How would your parents and fiancé feel if you quit your job and risked your life (by getting less access to good health care?) in order to in effect give money to a company?
You are more important than the company. And your future colleagues, will agree with this, I think. Companies are built to make people happy. Not the other way around.
(I think I a little bit can feel how you feel about joining the company and then later on they will know that you have this illness. And I think that they will think you did the right thing, when you joined. Assuming you don't over work yourself, but take time to rest when you feel you need to.)