I don't have a good experience with my Toyota dealer's ability to diagnose AC system problems - They told me it was the main pipes, but after changing them the system failed even faster and so they took it in again and decided it was the condenser and ac compressor, at which point I queried them on why they hadn't spotted that in the first place, to which they effectively replied they didn't check. They weren't even sympathetic or apologetic about it either, despite a fairly large sum of money being charged.
So I wouldn't put too much faith in their diagnostic abilities unless you have more positive prior-experiences.
I'd recommend finding a proper AC specialist, and check the Web for positive/negative reviews, but be warned - If it's been left gasless for too long, there is a good chance the residue in the system has turned acidic, and the longer it's been in there the higher chance the whole AC system will be damaged - I strongly suspect that's why my Mk2's system was riddled with micro leaks, the practically new condenser was (allegedly) leaking, and the AC compressor's internal clutch had (allegedly) failed.