I had researched this pretty heavily before deciding to go all electric/immersed element.
I read a thread that one of the guys that works at Cooktek (the better/best brand of cooktop) uses his 1800W 120V to brew 5 gals. You may try calling them and asking. I wanted to brew 15 gal, so I gave up. After 1800 W, it goes up to 240V. They then go up to 3500W, which are pricey but would probably handle boiling wort.
I also remember reading somewhere that the Morebeer kettles are magnetizable and would work with an induction element. Polarwear makes some really nice tri-ply stock pots (in addition to their kettles), but they don't have fittings, you have to make the kettle.
However, if Morebeer and Cooktek teamed up and marketed one of these with a 20 gal kettle, it would be a winner (they just have to make the heat levels more adjustable)... for the deep pocketed, that is.
Check out the video clip to see how big this thing is.
And speaking of adjustable, that's what I'd make sure to do if I were buying one. Make sure you get one with "100 heat levels" or whatever. One that has 12 heat levels would be hard to hit a temp. Even the 100 level ones have steps in 5 degree increments when you get up to boiling level. That's almost not good enough.
And another thing, some of these have an 'automatic pot size detector. I would just check things out with the size of your kettle and whether it's going to trip the computer up and not work right. Most of these can accommodate 12-14" pots. Not big enough for me.
The one I was going to buy (because of features and price) was a Vollrath. I'd check those out. It has timers (sweet! perfect for brewing) so people can cook sous vide, etc. I would make sure that if you wanted to use the timers some have that you make sure they go long enough (which is the concept behind Sous Vide, low heat/long hours in a vacuum seal). The better higher end of the Cookteks have this, but not the lower end (same element probably, different computer). The Vollrath did and was the same price as the lower end Cooktek.
Ebay is your friend, but I would be wary of the real cheappies out there.