It sounds like the chap knows enough about bearded dragons from what he has said. He has mentioned that they will require additional heating and lighting in their home, as the Thai climate does not suit their natural conditions in australia.
He also mentioned that the food items they need are different from those commonly available, which also sounds promising - it would have been far easier for him just to try and sell you one without any advice. The fact that he is trying to ensure that his pets are correctly carers for is again a good sign.
The different colours he is referring to are the different genetic 'morphs' that are selectively bred in the hobby. A google search for bearded dragon morphs should show you some of the colours available.
Finally I personally would much rather buy a captive bred animal than a wild caught one. There is less chance of disease and internal and external parasites. Captive bred animals also tend to be of better temprement. Although things are different here in the Uk than in Thailand, I would also be concerned that buying wild caught animals also ensures that the trade continues... if nobody bought wc animals then wild populations would be less threatened.
Just my thoughts on the matter.