What derailer hanger do I need? (screw on derailer is bent)
With respect to the bent screw, it's called the 'B-screw' and yes it is important.

The pivot in the derailleur where it attaches to the hanger is the 'B pivot'. The screw adjust the spring tension and controls how close the top jockey wheel comes to the largest sprocket. 

Assuming your derailleur was adjusted correctly you can leave the bent screw in place temporarily.

Online bike stores in the US don't seem to list replacement b-screws (or derailleur limit screws), but they would be a standard metric screw or bolt size (Googling bike discussion forums seems to indicate M4), so you can get a replacement of the appropriate length at a hardware store. A stainless steel one is probably a good idea.