I like your choice of switch location.

The Bronco kit looks like the way to go regarding price.
If you look around you may find the motors a little cheaper, but you won't get a switch, and need to make sure the shaft length and diameter are correct.

For the 24-volt guys, this type motor can be found for marine applications in 24VDC.
I have never seen any cheap ones though.
Example #1: http://cgi.ebay.com/ebaymotors/MARIN...ht_2886wt_1169
Example #2: http://cgi.ebay.com/ebaymotors/MRV-2...ht_2030wt_1169