Use PEX for the heat exchanger coil:
http://www.blueridgecompany.com/radiant/hydronic/453/rht-radiant-pex-b has 300 ft coil of PEX for $171 with free shipping.
This would remove the need to keep the temperature of the tank down in the 130F to 140F area -- you could set the upper limit for (say) 160F -- although at this point a tempering valve is probably needed.
Think about 3 solar water heater configurations:
Lowest cost, but still a high quality solution:
PEX collector (limited to high tilt $140
EPDM storage tank -- limit to 130F $200 (? might work out a better shape to reduce cost) (100 gallon?)
Coil of HDPE 300 ft $70 (consider 200 ft of 1 inch or even 3/4 inch?)
Cheap AC pump with $50 (probably pretty good life for 130F system
Steca dif controller at $100 (set top end for 130F)
$560 (maybe could get to $500
Fancier PEX collector?
Copper pipe over alum fins:
Copper/alum collector $190?
EPDM lined tank at 160 gal $200
300 ft coil of PEX $172 ( http://www.blueridgecompany.com/radiant/hydronic/453/rht-radiant-pex-b
Consider a PEX version with two levels of fancyness, and a copper version with 2 levels of fancyness.