I would have rather saw you take the dual 700 drill motor route as it would have been more powerful and much lighter. 900w vs 300w.

After researching the average blower motor, I have found they typically only spin about 1500-3500RPM. A Wiper motor suffers at much less than 250RPM which is literally un heard of in RC.

As far as 12v batteries you could purchase small hobby grade Pb's such as the following
http://www3.towerhobbies.com/cgi-bin...&I=LXL370&P=ML
http://www3.towerhobbies.com/cgi-bin...?&I=LXSK06&P=7

What car model and year was the blower taken from? Is the hull submerged drive? What size was your previous prop? What kind of speed are you hoping for?

Ryan