Plenty of people with bipolar disorder work full time jobs successfully and manage their disorders. Same with autism. It may require you to build some social skills to interact with coworkers, but it's possible.

As far as being overweight, yes, that can be an issue if there is any physical labor required for the job. However, perhaps wanting to go back into the field and do this kind of work could be a motivation for helping to improve your fitness?

Typically if a gap in work history is caused by illness, you can answer that simply by saying "from X year to X year I was undergoing treatment for an illness, and now I am recovered and returning to the work force." I would not give any details on what kind of illness, and they can't ask you. When I took off almost 2 years due to illness that's the phrasing I used, and I was able to find work. It does depend on how long the illness was, you might need to give a little more explanation about how you're hoping to re-enter the workforce - like having kept skills sharp by studying and such.

Hope this is helpful. Good luck!