I have three good URLs for you..but I don't have enough post counts to post them yet... I'll come back here and post them when I do have enough post counts.
Apparently peanut butter is a laxative, I've read, in the event he/she should swallow something it shouldn't have.
Ferrets LOVE Ferretvite - it's good for their coat.
Ferrets also love raw chicken bones - the marrow is real good for them, has a lot of nutrients in it.
As for what to feed it, I'd suggest Innova EVO's ferret. It contains the high amount of meat protein that they need to thrive: 30-40%.
Other ferret foods that I've read have been "Zupreem's Ferret" food, and "Totally Ferret - Older Ferrets". I don't trust Totally Ferret, personally, I scanned their website and their ingredients and wasn't at all impressed. Innova EVO contains the amount of meat protein ferrets need to thrive, as they are obligate carnivores like cats. Zupreem seems...borderline... they're an OK company in general, I feed my rabbit and my guinea pig on Zupreem, because it doesn't contain Menadione(deadly synthetic Vitamin K) like Oxbow and many other pet food companies do. But they're only being fed on Zupreem temporarily until my plants grow, then my rabbit and guinea pig will be fed an all-natural diet of plant greens and veggies, with little to no pellets offered.
The main thing I don't like about Zupreem, though, is that it contains corn ingredients - mainly corn gluten. Corn in general is bad ingredient in pet foods, but corn gluten is 10x worse. Pet foods should be gluten-free.
Overall, I'd suggest Innova EVO's Ferret would be your best bet. And if that's a little too out of your price range, ferrets can be fed on cat food, and in the event you'd rather do that, "Wellness Complete Dry Cat Food" is a good bet.
I can't post links, but google search "wellness pet food", then click on the "Cat Food" link on their website, and then click on the "dry formulas", and then click on the "Chicken" link in the side bar.
For Innova EVO, google search Innova, in the side bar click on Evo, and scroll all the way down to the bottom to find their listing for ferret food and click on the "Learn More" link.
Whatever dry food you do, make sure the meat content is high, and make sure the ingredients are chicken or turkey-based, apparently ferrets aren't all that fond of fish-flavored foods.
EDIT: I read on one of the web pages that I'd love to link you to, that female ferrets should be spayed before their first heat cycle. Apparently, if they aren't, they develop anemia and don't make it past a year old. I'm sure another ferret-lover could verify that for sure; but that's just what I read. Still, that might be something to look out for should you get the little ferret-bugger at the pet store.
Verify if it's either male or female.
That's another piece of information I can offer you, also found on one of the web pages I'd love to link you to - males and females can be distinguished not only physically(males feel heavier in your arms than the females), but also in their litter box. In a litter box a male is using, everything is sorta scattered, while for a female, everything is kind of neat. Females are also more typically high-strung and don't like to be held as much as males do. Again, I'm sure someone else can verify that.. but it might help to verify, at the pet store, what gender the ferret is at first glance. I know at least in any pet store around where I grew up at, the ferrets were litter-trained, so... there you go.