Last time I was riding (trekking bike), I had a problem with my derailleur. I discovered that the cage was detached from the body (with the attaching screw missing) and was bent in half, hanging on the chain. Now one option is to go to the LBS, but I would prefer to learn how to replace (and choose) the derailleur myself.
Current derailleur: Shimano Alivio, RD-M410
Cassette: Shimano Alivio, CS-HG50-8I (sprockets 11 to 32)
Should I do it myself? What should I look at when I want a more durable RD?