It will be cheaper to build a rotary phase convertor. If you want to go single phase The Surplus Center had a 7 1/2 farm duty for $650 a couple of months ago. I'll see if I can find that link again.
EDIT: here is the link to the motor:
https://www.surpluscenter.com/item.asp? ... e=electric
Here is the contactor you will need to run it:
http://cgi.ebay.com/GE-CR305L1-Size-2-T ... 4a96a99ca9
Don't worry about modifiing your RCM and all that crap, just hook the output of the current three phase blower contactor to the coil on the Size 2 contactor. Basically the size 2 becomes a "slave" relay. I wouldn't worry about a heater either, since that single phase motor has it's own overload protection built in.
BUT, since your motor guy is going to hook you up with a rotary phase convertor (lucky), I wouldn't worry about none of that. The only fun part will be dragging that 25 horsepower phase convertor around.