As a soldier, I can tell you that American MRE’s are fantastic! …When you first have 'em. They get old when you’re forced to eat them over and over again. Some of them are great, and some of them not so great. Really, the quality is highly dependent and personal taste, but in general if you have to eat them cold, it’s not a great meal. All of them are sanitary hot or cold, but you generally wanna do things like scrap the preservative meat gel off the burger patty before eating it cold. And they aren’t notorious for low fiber, but for constipating you. They also have a ton of sodium in them, which we’re told helps with water retention.
Honestly, if you want something more realistic, I think the balance for MRE’s should be that they disassemble into multiple little meals, which have various cooking times, enjoyment and (once cooked) spoil times. If you get a spaghetti, it’s bearable cold, and hot it’s pretty good. If you have a cheese omelette, it’s horrifying cold and heated up is still terrible. Add in tobasco/salt/pepper packets to either increase enjoyment or minimize disgust.
Net result: MRE’s split into a bunch of small meals, which are balanced out to be roughly the same nutritional value for each MRE. Some are mostly good cold, most are really bad cold. If you don’t heat them up, they’ll last for a year+, if you heat them up, you’d probably better use them in a day or two. Overall the MRE can either be used to eat quickly and just get some calories, at the cost of morale, or most items can be heated to make them more enjoyable, which takes a significant bit of time. Optionally each MRE comes with a chemical heater bag and each main entree (typically two-three per MRE) comes inside a cardboard box, which could be used to cook them via fire.
Far as the heaters go, they take a small amount of water to use, which is not packaged with the MRE. I tend to take a small mouthful of water and then spit it in, since that measures it out pretty well (if probably not all that sanitary). The hard part of the heaters is using them for more than one thing. There are all kinds of tricks where you cram parts of the meal and the heater into the cardboard boxes, or slip them inside the heater bags, etc. Takes a little bit of experience to get them down.
Also, the heaters can be used to make MRE bombs, which I’m sure you can find a video for on youtube. Basically a short delay noisemaker at it’s basic level. Would be useful for tossing away to distract zeds who can’t see you, like during a night raid.
Aaaand back on topic, I think that mostly the issue of what to use as a basic currency will resolve itself. It’ll end up being an object that most NPCs will accept, which is light or no weight or volume and can be easily transported. And something that’s decently common and separable into small denominations so it’s easy to the trade even. Cigarettes seem like a good choice for this, as do most drugs/medical supplies.