It's not at all necessary to stick with the Bernina brand for your serger; there is no feature interchangeability between a serger and a sewing machine.
IMO, you definitely want differential feed. Probably most sergers have that now, but back when I bought mine it was optional. It *really* helps with sewing on a variety of stretches and eliminates the need for a third hand. :D
I did not buy one with a coverstitch and have regretted it ever since. It's really easy to make t-shirts and other types of tops on a serger and the coverstitch provides a very professional finish. Without it, a lot of my stuff looked more on the "homemade" side.
A lot of people really like automatic threading -- where the air blows your threads into place. Threading is the most tedious aspect of serger use, so this might be worth looking into. It's a feature I'd like to have.
Before buying any serger, check the reviews at
http://www.patternreviews.com . Check Craigslist, eBay, and your local sewing machine dealerships for used machines. Check the reviews before purchasing, and I would probably also check the model for "completed listings" on eBay to see what they actually sold for there.