I absolutely do not recommend a coop specific to chickens, if you're going to spend money.Our biggest challenge at the moment is trying to keep the cost of building a coop down. The materials to build a medium sized coop seem to run about $600 - it would be nice to get that cost down. We're going to look into the Amish to see if they'll build one for a reasonable price.. we've been pricing out materials from the Home Depot vs. several different plans and OUCH to the cost! I did see several coops at Tractor Supply, but they are really tiny!
A nice garden shed will work just fine and can be converted back if you decide not to keep chickens. Coops specifically built for chickens are not easily sold. They are hard to move and most don't have the means or way to do so.
Check out the coop section, google coops and go from there. There are many great and cheap designs.
It only makes sense that if they're selling it to you for $600 they didn't spend that much on materials to do so.