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To answer your first question, I would expect higher amounts of sodium and preservatives which are not great for your health.
Given your parameters of only microwave and almost no fridge space, and expense here is what I‘d recommend:
Canned sardines and mackerel
Eggs if you have the space
Oats for savory and sweet oatmeal (not premade mix)
Fresh spinach or romaine/green leaf lettuce (not premade mix)
Chickpeas
Avoid buying premade mixes, that’s where costs add up. For example, premade oatmeal mixes are easily twice as expensive as rolled oats in my area, and less healthy.
check out local soup kitchens or food pantry’s. There’s no shame in a little extra help. They usually have excess items and need to give them away. You’re not taking away from someone who might “need it more” because you’re in need as well.
Have you considered looking up raw whole food prep that requires no cooking/heating?
To answer your question going mostly canned you will be consuming a lot of sodium, having too much will increase certain health risks.
You can counter-act this by increasing your potassium intake (bananas, avocados, spinach, mushrooms), which help reduce sodium levels. Also remember to stay well hydrated.
You’ll be fine! Just watch your salt intake. Tuna, microwave rice and tinned veggies for the win! There’s loads of healthy tinned options. Also avocado on seed bread. Peanut butter wraps. A variety of nuts are great too. We go camping all the time for long stretches without cooking or fridge. You got this!