Here's my advice: consider trying one of the products that seals the leak, so you don't have to replace the evaporator. Cliplight Super Seal Premium for instance.
Cliplight - Super Seal Premium - The Classic Auto A/C Sealant
If your evaporator leak is slow enough, like it takes a couple weeks or longer to require a re-fill of the r134, a sealer will probably work for you.
Research required! Study up. I think the steps needed for high rate of success:
(1) Empty the system - recover the refrigerant.
(2) Replace the filter / drier and any other components that you're planning to replace, fill system with dry nitrogen and small amount of tracer gas like r134, pressurize to ~200 lbs, check for leaks with a sniffer. No leaks other than the evap? Proceed.
(3) Draw a vacuum, follow the directions to fill system with Sealer and r134.
(4) Keep the car running with the AC running on high for the required time to circulate the sealer - I think it is a minimum of 30 minutes.