A new AC unit could lower your electricity bill. Also, see if your utility company offers any rebates for installing a new energy efficient unit. See if any federal tax credits are worthwhile http://www.energystar.gov/index.cfm?...rebate_locator
An HVAC contractor friend sometimes replaces the entire unit because the pan leaks. He hates to do it, but sometimes it is the only thing that makes economic sense. You might try calling a couple HVAC companies to see if they can just replace the pan (or entire coil assy) and if it is worthwhile.
I don't know if it is significant, but the condensate drain typically has a trap to prevent air leaking through the drain. Your auxillary pan won't prevent air from leaking through the hole.