You will not want to use PVC for your blades as you will get significant tip erosion and be replacing them at least annually. Coating them in fiberglass will make them last signifantly longer.

To generate 600-800 watts of power with a direct drive DC altinator in low to moderate wind you will want at least 6' blades giving you about a 12' rotor diameter. You will also need this to spin at least 25 rpm, this means your tip speed be traveling about 9 MPH almost continiously. Soft PVC does not hold up well to this. At higher RPMs I have seen PVC blades detach themselves and become missiles.

I have worked in the wind industry for several years and manage several large industrial wind farms. I have supervised and judged many technical college and university competitions for building small wind turbines. I have seen a lot (almost all) of the PVC blades fail.

That being said, if you have a school nearby with a renewable energy or electro-mechanical program you might be able to get students to do a project like this for the cost of the parts.