I also liked Tim Dunigan in the pilot, and thought he was quite believable as a con-man. But after that, the show wasn't really going for realism. So having a good looking guy like Dirk turn up, smile and make women swoon enough to give him what he wanted, fitted in more with the campy nature of the show than any realism would've.
Dirk was good enough, there's no real need to consider other options.