This is a HUGE job. Only you know whether or not you want to tackle it. As far as time spent, it probably rivals an overhaul, or at least a valve job for time spent.
I'm sure that it goes without saying, but you want to BE SURE the evaporator is leaking before going to all this trouble. The best way to do that is put in a can of r134 UV dye. Then after a day or so, use a UV light to look for a leak. If the leak is indeed the evaporator, it will be difficult to get at with the UV light, but you might get lucky.
Another thing, if this system is already changed over, and the leak is small enough to hold a charge for a few months, then just recharge when it is low. R134 is cheap. You will need to take into account that you are apparently losing oil with the leak IF your tech is correct in his suspicion.
If it is a slow leak, it would be worth purchasing a set of r134 guages and a can tapper.
Best of luck,