I don't remember their whole roster, but I'm sure they get a huscarl unit. I'm sure I've had sucess (and failure) as Northumbria, but its been so long since I played them that I don't remember what I did.

I suspect its a good move to destroy your monestaries to make your land less attractive to the Vikings and give yourself a fatter treasury at the start. Probably its also a good move to secure an alliance with the Vikings if at all possible, and be cautious about attacking allies of the Vikings, at least until you have some sort of fleet to fend them off, or they run out of money at which point they become much more docile. I expect its a good move to invade Mercia as soon as you think you can pull it off because 1: they would become a horrible neighbor once they got huscarls, and 2: they have great farmland you can use and 3: with any luck you should be able to corner and ransom their king one or more times.

Also, don't they get +1 valor woodsmen from somewhere? I think you can get an additional +1 valor for building upgrades and make them into a decent fire and forget flanker.