I bought a MS 261 C two weeks ago. It was it was $569 plus tax. I got a 3/8 lo pro bar and chain on. They had to change the sprocket plus I kept the .325 sprocket that comes on the saw. They also had to remove 2 drive links from the chain. Most dealers will do better if you tell them you want a cash price.
I don't think I'd want a 20" bar on a 261. I does not have enough power to pull it IMO. The dealers here had the 261 equipped with a 18'' b/c. The saw was balanced nicely with the 18''. I was worried mine might be too light on the nose with the 16'' 3/8 lo pro on it but it is balanced good.
If you get a 261 be sure it has a 3 position switch because that makes it a Mtronic model.