Most of your investment will probably be in lenses depending on the type of shooting you do.
I know plenty of 'pro's who shoot with 20D's, or 5D's. You don't need a 1DSII or whatever your brand preference equivalent is. Being professional means photography is your profession, and generally that you have been trained or have trained yourself to the necessary level to shoot like a pro, this isn't necessarily all about the shot. A professional wedding photographer might be crap at landscapes, or perhaps just not very creative. It wouldn't stop them being a professional.
You can shoot landscapes as a profession and never concern yourself with which (D)SLR to buy. Or portraits for that matter. The tool should be suited to the job, a camera is a tool, just a hammer, nothing more. A panel beater wouldn't try to shape a new fender with a carpenter's claw hammer.