Is the Echo CS-2511T good for climbing and tree work?

Yes. It is a top-handle saw designed for arborists, light enough to use one-handed in the canopy. It is not intended for ground felling by inexperienced users.

How does it compare with a rear-handle saw?

It is lighter and more compact than most rear-handle saws of similar displacement, but the top-handle grip is less stable for cutting at ground level. Choose it for in-tree work, not firewood bucking.

What bar and chain does it use?

It comes with a 12-inch bar, which suits limbing and small removals. A longer bar would upset the balance of such a light saw.

Does it need mixed fuel?

Yes. It is a two-stroke gas engine, so it runs on gasoline mixed with two-stroke oil. That is one upkeep point battery saws avoid.

Is it safe for a beginner?

It has a chain brake and chain catcher, but top-handle saws are best learned under instruction. Beginners are usually steered toward a rear-handle saw first.