Many DF systems use Economite burners. Google them. I am working with a Powerflame burner (mainly because of good local service) and so far it has been fine. You can Google them as well. I am using a X4-400 model on a 7 BBL kettle.
on a side note, make sure the burner is UL listed. If it is US made you will have no problem but if you get one from China (for example), your electrical inspector may give you grief without a UL listing...
If you want a 700,000 BTU burner, I've got one I'll make you a deal on
Blank Slate Brewing Company