Even if you want to buy one I wouldn't touch any of the current commercial ones in comparison to buying one of Rex's burners:
www.Hybridburners.com
The burner is the heart of the forge, the rest is just a metal box with some insulation in it. Or a stack of fire bricks. I'd hate to see a guy lay out a pile of dough, and when it comes to getting the unit out of the box, he is still doing as much screwing together or parts as if he had made his own, and at far higher cost for a less well made product.
I haven't tried one, but if I wanted to drop some dough, and I was going to use flux, I would at least consider buying The zawata shell and a Rex burner. Comes out around 500, and it should be to die for.
http://tzknives.com/forgead.htm
But the real bottom line is that if you can weld, you might be better off just getting the burner. There are different styles of forge for different puposes, and you can wack together a new shell to handle differnet work as the mood takes you. Want to temper a sword, or just make plane blades? Different shell. Want to do wraught iron? Different shell.
I almost bought a Merkel, but I'm glad I didn't, not that they are bad or anything, but it makes more sense to put your money where it might count. There is lot's of very cool stuff that might be more worth your while, like a pyrometer, laser heat gun, rex burner, or Zawate, vs these 1920 style gas forges.
Safety wise. Try this. Get your propane torch. get a coffee can with the top off put it on it's side on a flame proof surface. Punch a hole large enough for the torch in the side of the can. Light torch, and stick through side of can. That's as exciting as it gets. You are not going to blow up. Take the torch out of the can before it gets too hot, it isn't insulated, though if you did insulate it, you would have a forge. I can see people not wanting to make their own burners, for safety and efficiency reasons, but if you can buy a ready made burner for not much more than the cost of a Bernzo torch, why not go that route?