I wouldn't hurt to call around to the vets in your area and ask about a payment plan, beachin. Seriously--I think the vet visit is a must at this point. Even though funds are tight, I'd strongly suggest at least a urinalysis and a stool analysis. Unfortunately, because you don't have a vet right now, there will likely be an office call fee (once you're an established customer at a vet, they might allow you to just bring in a stool sample and some free catch urine if you can get it, but as a new customer, the vet is likely going to insist on seeing your cat the first visit).
A urinalysis would pick up diabetes (sugar spills into the urine) and might be able to pick up kidney problems (specific gravity can tell you a lot about the kidneys). Some worms are difficult to find in the stool--and some parasites are blood-borne so they wouldn't show in the stool at all. But you can often rule out things like tapeworms, hookworms, etc, with a simple fecal analysis.
Milk is probably not a good idea. I don't have a lot of cat experience, but as I understand it, many of them are lactose intolerant.
Wet food is better than dry for cats, I know--and the less grain in the food, the better. Cats are obligate carnivores, so they need meat. Many cats do really well on a raw meat diet. I'm hoping some of our cat gurus will sign on later to give you some advice.
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