What you need to do is get the system charged up and have them add an infrared leak detection dye to the system. It could be that you just have a tiny leak that may or may not need attention. A lot of cars leak refridgerant, and will still go two years between recharges. With the dye in the system you can not only easily locate the leak, but also gauge its severity. Once you have this information you can worry about a possible evaporator (which is behind the dash). You won't damage you A/C system running it now as once the refridgerant pressure sensor detects a lack of charge, the compressor will not turn on at all, hence the EC light staying on.