First, it's not at all difficult to buy and install a nice mantle (or mantle facade) to dress up your fireplace.
Fireplace Mantels - MantelsDirect.com
However, if you're thinking of having the fireplace and facade stand out from the wall a foot or so, that's something entirely different.
Yes, you can swap out the doors and brass trim. To say you can do it "easily" might be overstating it a little.
If you're doing the work yourself, I'd think you could
really update and improve the look of the fireplace for $1000 - $15000. Maybe less, actually, depending on what you decide to do.