I'd buy it! You can pick up the windows and stuff for pretty cheap if you shop around. Maybe look and see if there is a Habitat for Humanity shop near you.
I got really lucky, now mine is metal and not wood, but it's 10x8 and in almost perfect condition for $50.00
I have seen the nice ones that your looking at, and they are costly. I would say it's a good deal. If I had found a deal like that, and could buy it, I certainly would.
I do think that it would be cheaper if that guy would build it for you, just for the cost of material. I would get a material list and call and get prices on the material and then go from there.