I joined the military in 2010, and weighed in at 82kg, after completing basics and corps training I was 69kg and looking like a wet cat. So I played around with so many programming and eating methods, that I stagnated and kept rebounding. After eventually reaching a sticking point of 76kg, I decided to give DC a go.
Being in the military, getting enough food to provide a caloric excess was quite difficult. Milk gives me the shits so I avoid it as often as possible. But somehow, chocolate milk as well as ice cream manage to stay down pretty well.
There are cheap options, but you have to be prepared to change things up as well to avoid allergic reactions.
You'll have to buy bulk, so it will be cheaper. Keep on your list:
Cereal (kids cereal is awesome for spiking insulin and makes me feel satiated before bed)
wholewheat pasta/instant noodles
hot sauce - goes on everything when shit gets boring.
dried meats - here in South Africa, we have biltong, not sure what your alternatives are.
vanilla ice cream.
chocolate drink powder
Regarding weight gain, don't be afraid of a little extra thickness and blubber. pack your food in. I found that having 4 slices of bread, along with peanut butter and syrupt kept my macros high and placed me in a great zone for weight gain. I eventually managed to push myself up to a nice and thick 92kg. although my cheeks did show I was loving life! I'm now cut down to a much leaner, more functional 87kg and train for MMA, although, I still love my high rep leg presses, and have kept the strength gains from DC.
The trick to staying lean is to slowly introduce bigger portion sizes and let your body adapt to the new feed intake. Up the macros too quickly and your rectum will start to feel like a balloon that's been blowing all day.
If you're not supplementing with protein or taking a weight gainer, you can make your own by blending, some eggs, oats, vanilla essence, milk and peanut butter. great morning shake and post workout shake.
Drink in excess of 2litres of water prior to working out... it really maximises your pump, and is quite excellent.
Do steady state cardio religiously, and use a belt to support your lifting. You'll have to look into some prehab/rehab strategies for your back.