My son's bike is an older Schwinn Moab mountain bike outfitted with Shimano grip shift shifters and Shimano SIS derailleur (7 speed rear, 3 speed front).

The rear cage got severely mangled and I need to replace the entire derailleur. I am guessing it is a 'medium' length but not sure.

So my questions are:
1. Where on line can I get an exact replacement (if possible - bike is about 6-8 yrs old.

2. Given the cost of the bike, I am guessing this is a fairly low grade derailleur...what would be a reasonable upgrade to replace just the derailleur? I am guessing the original cost between $15-20 USD....Maybe something around $30-40?

3) If I got an upgrade, what features would it have or enhance over the stock derailleur?
For example, would it make the gear changes snappier? Less chain bounce when changing, etc.

One thing I noticed is that it seemed to require constant periodic short term adjustments. It seemed that after 3-4 days on the trails, had to adjust hi/lo limits...

Thanks for your help.