I am from the UK, and came over on a working visa about 3 years ago. You will need to claim medical issues, but the questionnaire you will need to fill in is very broad. I recently had to have a medical due to visa changes about a year and a half ago, and it took 5 minutes! The doctor only went through the questionnaire with no other questions. I had been hospitalised frequently both here and the UK. But because the questions are really non-detailed, it really didn't come up much. I guess my advice is just to answer the questions, but don't elaborate. They are only interested because they want to see how much you may cost the government; for example hospitilization. You will need private health insurance, and that obviously would cover certain costs.
because you have never needed hospitilization and are stable, I cannot see a reason why the visa would need to be declined. Generally visas over here are a nightmare, and because of ongoing changes, prove to be even worse lately. I would do it through an immigration officer, but do your homework when choosing one as we have had some bad experiences. The problem is if you do the visa yourself and accidentally make a mistake on the application- it would probably be declined. And you seriously don't want that. The immigration people don't look into reasons why it would have happened, and just see it as a decline, which would seriously hinder other applications. Also with the current climate it has gotten seriously complicated.
What visa would you be be applying For?