Air Conditioning - Condensate Leaking - No Evap Coil Access Panel

I have a 25-year old Carrier split system which has a 5-ton rated cooling capacity. The cooling coil section (Carrier's 28 Series) is mounted on the top of gas-fired furnace, model 58GP. I live in the coastal So Cal and I have used the cooling mode on average 1 - 2 weeks a year. The system has been operating without a glitch for years. The only thing I had to change was the PCB due to a faulty relay. Because of our unusually early hot and humid summer, I had to turn on my A/C a few days ago. Soon I discovered that my garage floor was becoming flooded around the furnace pedastol. I shot the A/C unit immediately and waited a couple of days for the unit to get dry. I disconected the drain connection and used my shop vac to clean the connection. I found no significant dirt in the drain connection, only traces of corroded metal (max 1/4" size). I restarted the unit and after a few minutes I saw a small stream of condensate flowing out through the drain and simultaneously through the furnace, dripping profusely on the floor below. IMO, it looks like I have a problem with the condensate pan. Unfortunately, the coil is encased into a duct that has no removable panel. Facing the duct, both drain and freon conn's are located on the front panel, close to the left edge (I wish I could inlude the picture but I do not know how to get around the posting rules). I also do not know if the coil is A or N type. I would appreciate any feedback on the following concerns/questions so I can decide what to do next:
1. What tool should I use to cut out the opening and where and how big the opening should be?
2. How difficult is to replace the condensate pan? Is this too much for the DIY undertaking?
3. Do you know of any web site that may provide more info on the condensate pan - pics, drawings, and installation/replacement manual, etc? (I had no luck with Carrier's web sites)
4. If the pan is not replaceable, can it be repaired, i.e. sealed and with what?
5. How common is that one has to install a new coil section because of a pitted condensate pan?
6. Replacement of the coil section must be costly and requires the licensed contractor. Any idea on the approximate cost?

I would appreciate your early feedback (it is very hot here) and many thanks in advance.

You can post pictures on photobucket or similar photo hosting site & provide a link here.

Use "aviation" snips (not tin snips) to cut the sheet metal. Just cut open the whole front of the coil box (plenum). Any decent sheet metal shop can make a pan for you. I usually pay $60-80 for one but with you being in southern CA, don't take that as being carved in stone.