Fruit is good, but not too much, it can not be their primary diet.
Most enjoy unseasoned/unsalted chicken & beef. (Some feed them uncooked, but it isn't recommended) Yes, bugs also, such as crickets. Some are even fed day-old chicks and baby mice (pinkies). Feeding them this is okay, but don't let them "stash it" or "save it for later". Whatever they do not consume you should remove it from the cage.
You can give the eggs raw, but only one per week.
If you are cooking, I would go with scrambled, with the shell crushed up. Do not add anything, they don't need anything extra.
Peanut butter- A little bit. They make peanut butter treats, so they can be fed plain peanut butter, just not alot. Maybe just a little bit on your finger. If I were you, I'd just buy the treats.
The housing arrangements sound fine.
& I know exactly what you mean. We found a ferret also before. Poor thing was just skin and bones. Didn't look or feel healthy. We really didn't think he's make it, but happily- he did. (: And he lived with our other 2 ferrets for many many years. (:
I know, if you search online about feeding fruits, many people will say no. And you should not give it to them in big quantities. But upon finding a ferret, we took him to our vet, which insisted upon us to feed him fruits. Not lots, but 1 grape a week, and 1 piece of banana every 3 days. We did once a week and a little bit of dried fruits, as recommended by our vet. The vet told us to continue this, add some meat, and also- Cat food. Cat food is usually frowned upon, because, it makes them gain weight quickly. Which for any other ferret, this is a bad thing, But for an underweight ferret, good. Feed her catfood (Dry.) for about 1 to 1 1/2 weeks, then slowly start mixing ferret food in there. Like, one day 90% cat food, 10% ferret food. The next day 80% cat food 20% ferret food, and so on.