If straight bars are what you want, forget about cyclocross bikes. They're all drop bars and integrated levers. Converting them to straight bars means you're throwing away the most expensive part of the drivetrain--the integrated levers.
Better to look at flat bar road bikes that can take wider tires. Then you're just changing tires and your saddle.
Bear in mind too, that as your position changes on the bike, and as your ride distances increase, that fat, puffy comfort bike saddle can become a real source of woe.
Here's a sampling of flat bar road bikes:
No, I haven't investigated which ones can take fat tires. This is just to start you off.
Finally, you may want to reconsider drop bars. Ride a few bikes with them. You may find they're not at all what you think, particularly those with "relaxed" geometry..