You are so not alone, while your own particular circumstances are maybe unique, I can relate to physical limitations, compressed discs and a bulging disc, one knee surgery done, another looming, so exercise is difficult.
I have been on medication for depression on and off for a lot of my adult life.
OK, I know you don't want to hear the next bit, but you don't want to lose weight quickly, you want to lose it for good. I did the losing weight quickly bit, lost over a 100 pounds, but I put it back on just as quickly, which of course made me depressed all over again, yeah I'm sure you know the cycle.
I have just started on a get fit scheme again, and this time I'm going a little slower. For exercise I have ordered the 'Yoga for the Rest of Us' DVD, as a way of starting to move again and strengthen the muscles and joints, I also need to get my butt on the stationary bike.
As to the cost of good food, I know it is hard, but junl food is actually expensive wen you add it all up, especially the cost of what it is doing to your body.
The thing you could consider is making little changes, start by cutting out snacks, drink more water, then make breakfast a healthy meal, drink more water. When you have trained yourself to no snacks and a healthy breakfast, then change your lunch to a healthy meal, and so on.
If you are better at big changes start researching a diet that sounds good to you, one that includes food that you like, and doesn't ask you to eat foods that make you puke
You also need to work out your body type and what will work with you to lose weight. I know from much trial and error that I can lose weight really easily on a high protein low carb diet, but I struggle to maintain it, because I am a carb addict. Currently I'm on day 6 of a vegetarian fast, and so far it's about the easiest thing I've ever done, unlimited fruit and veggies, some nuts, pulses, and whole grains, no meat, animal products, no yeast or sugar or sweeteners. I'm staying off of the scale, but I feel a lot better and it is making me more mindful of the hidden sugars in processed foods we eat, and to the mindless snacking I can indulge in when making food for the family.
This isn't a diet for life, but I think it is a good start, and when I start adding meat products I will be more mindful of what I'm actually eating.
You are not alone, and you can do this.