You can feed them gp and/or chinchilla pellets, as long as there are no sugars (cane sugar, molasses, etc) in it. I've searched the bags of every single gp and chin food at Petco, Petsmart, Petland and a local pet shop. The only one I have found that doesn't have any is Gertie Guinea Pig (there is a slight ammount of sugars, but someone contacted the company that makes it and discovered that the little yellow banana circles are the sugar containing ingredient, so if you pick those out of the bag, the rest of the food is fine. One of my local pet shops also carries Brisky degu pellets, which are available online at www.brisky.com
I feed my guys a mixture of the Brisky and the Gertie Guinea Pig, because the Gertie is cheaper (and available at Petco and Petsmart right down the street) and the shop that sells the brisky is a little ways away.
The only other recommendation I have is to make sure the cage you buy does NOT have a plastic bottom. We had to house our two guys in our plastic bottomed rat cage for a weekend (the rat was moved to a smaller cage for those few days) while we were waiting for their cage to be ordered. They behaved for the first two days. On Monday, I called the guy at around 11 am to ask if the cage was ready. He said to give him two hours to unload the truck. In that two hours, the degus chewed all the way around the plastic bottom of the cage (near the top, just below where it meets the metal bars). Fortunately, Milo is a total homebody and has not done any more chewing where they started. No doubt in my mind that the degus would've been out of the cage by the end of the night.
I haven't had my guys long, so I can't really comment on diseases and stuff like that. I've heard that they need vitamin C like guinea pigs do, so I buy guinea pig vitamins to put in their water. Not sure if it's necessary or not, but I figure better safe than sorry.