Hi Ben,
i can't advise on the access course but if nursing is what you really fancy then go for it. I can even pretty much assure you, you won't be the oldest on your course as a lot of people come into the profession later in life. I've even met someone in their 60s retraining as a nurse!
As for your occupational health assessment just be honest. Being overweight won't matter although there is always ongoing debate about health professionals and weight but that's for another thread! Be honest about your past problems with depression. They will just be looking to make sure that at the present time you are physically and mentally able to carry out the role of the nurse. Depression is only likely to affect your health assessment if it is an ongoing and present thing, in which case I would say to hold off until you're mentally well.
Hope this has been of some help!
Aurora