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

Upgrades ??

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.