As someone who dogsits four bernese ranging from 4 months to 4 years for a friend, I hope I can give some insight.
The bernese is a breed ranging from large to giant, as a poster above said they had a 30kg 9 month old in, but one of the dogs I sit for is 4 months and already 23kgs. The males tend to be on the larger side. They are very prone to hip/elbow problems, cruciate damage and cancers. So insurance is a must, along with a good food and ensuring the dog comes from a good breeder. Be very careful about the breeder, and I cannot stress this enough as there are a lot of bybs now dealing in the bernese with their growing popularity.
They are prone to being shy if not properly socialised, so socialisation is another definate must. They are generally excellent with cats, small dogs and other small animals, but again early introductions are a must. Like most big breeds, they know how to fit themselves into a small space so dont take up too much room indoors. They shed very heavily twice a year as well and need a good brushing to remove the dead undercoat. They can also mat quite badly behind the ears and underneath so need someone who is happy enough to spend time grooming them. The average lifespan is 8-9 years though some can live as long as 12-14, then again, others have passed on as early as five
They may indeed be a working dog, but I would debate them needing a lot of exercise. Five minutes per month of life is the best way with bigger breeds and I wouldn't walk them for more than an hour a day. As adults they are happy to lie around and snooze for most of the day but as pups they can be quite high energy. In saying that, do not overexercise the puppy. The exercise they get from running around in a garden, stopping and starting as they like is nothing like a steady walk and walking too much will cause orthopedic problems.
Make sure the breeder you go to has had the parents hip and elbow scored, that the parents are both IKC registered and if they're a member of the Bernese Mountain Dog Club of Ireland all the better. The club is probably your best bet for getting started, they can reccommend a good breeder. Keep in mind, you may need to wait for a pup as most breeders will have waiting lists.
Best food depends on how much you want to spend on dog food but I would definitely reccommend that you find one with no preservatives/chemicals if you can. As I said, cancers are prevalent in this breed and they are the number one killer of Bernese Mountain dogs so a good high quality food will suit him best.
Cant think of anything else at the moment, but ask away if there's anything else you want info on.