My dad has a 1641, he has the Moded 450 0r 451 45 inch blower. It's an awesome snow removal tool. With a little luck and patience you can probably find a used one for around 300 to 500 dollars. I can get the model # for you but if you take the earlier post up on his offer, you'd get that and more info. I think they run about $1300 new. On a gravel drive a blade might work better?
Curt