[help]Could use some advice

I don't quite know where to start, I work three jobs all desk work. This holiday season has not been kind to me. However all my weight started when I had undergone treatment for cancer, I basically had to change diet after I had surgery and I ballooned up a good 75-80lbs. My energy level is always around a 1-2 on a scale of 10 and I have to eat some carbs in the morning for digestion of the supplements I have to take. The energy thing is a medical factor and from the weight as well.

Are there any suggestions you guys can make towards diet and exercise?(I will cover my current diet below as well as daily routine)

Since I have been in remission for a while now, my diet can somewhat return to what I want it to be, other then needing some complex carbs in the morning for B and iron supplements. I have cut out all sodas(diet or otherwise), I stopped eating refined sugar for the most part other then the occasional piece of chocolate. I stay away from processed fatty foods when I can. I have quite a restriction on what I can spend on food and wanted to get some tips on what I should spend my $100-200 food budget each month on?

As far as exercise I have a dog and he has been kicking my butt. Not to mention that he has helped stall the weight gain but he is losing his extra weight as well(pug). I have time for my two walks with him daily and not for much else between my dogs. I was thinking about getting a weight loss ball for sitting on while I work on my computer. I'm roughly 300lbs and I wanted to know if there are any good brands out there that wouldn't pop(and I have looked at the local stores to no avail and the clerks haven't been of help). Would that excersize ball be a good thing to get? I get some breaks in the day and have my own cubical are there any stretches or low impact(so I don't draw attention) exercises that anyone could suggest? By the end of the day I would love to lift weights and or do situps but I have enough time to shower and get ready to crash for 6-8 hours before I start again.

I know you guys probably have heard a lot of these pleas for help, I just wanted to see if anyone out there has gone through what I have? I have been chunky my whole life, I'm ok with chunky I just don't want to be 300lbs anymore. Even if I get down 50-60 lbs I think I could smile when I see myself in the mirror again.

Thanks for your time.