As to which lens you should buy, my general advice would be take a look at the kind of photography you have done in the past. Buy lens which fit with your prior usage. You should of course adjust this somewhat since you prior camera may not have been capable of some types of shots. But there is little point to buying a 300mm lens if all you ever shoot is wide angle shots.

In general if you already have some glass, I would shoot with that until you find it limiting in some way. When you know how you are being limited, then buy something to fill the void.

(RP)