I found 5 solar panels (hydronic) for sale in CO for $100 each. All the tag says is Olinsolar model # II. I couldn't find any info doing a search. Probably out of business. Here's the link:
olinsolar panels
They have 1/2" inlet and outlet. They guy says they don't use the conventional manifold/riser design nor are they serpentine. He says its a kind of 'spider-web' pattern inside. He's going to scrap them out on Monday. Says he can get $90 each for the copper. The design I was thinking about basically has 3/4" manifolds with 1/2" risers, 6' high x 20' long with about 35 risers. The cost of materials to build it would be a little more than what the Olin panels are for the same square feet. From where I live it would cost $200 in gas and 10 hours to go get 'em and if I build 'em and something goes wrong then I have no one to blame but myself.