I've been thinking (fantasizing) about the same thing, but a barrel (31 gallon) biab nano-brewery setup. I'll walk through my thinking and you (or anyone) can correct me where I'm wrong.
Electric immersion heating elements are 99% efficient, so the first question I have is how many watts to heat about 45 gallons of mash water from 65 to strike temp in 30 (or so) minutes? (I don't know yet myself)
Then how many watts to heat 40 gallons of first wort to a boil in 30 (or so) minutes.
I'm hoping I can do that with 18kw (or 3 6kw elements).
If so, I'm thinking of getting a blichmann stainless steel 55 gallon kettle for $800, drilling 3 holes and installing 3 immersion elements with integrated thermostats from bubba barrels.
http://www.bubbasbarrels.com/6-kw-immersion-heater-w-thermostat
They're $300 each so that's $900.
They are 15" long, so I have to stagger them vertically, but I'm thinking I can get all 3 in the bottom 3 inches of the kettle and then put a false bottom above them. The false bottom is to keep the bag from touching the elements. In theory the elements will never be on when the bag is in the water anyway, but better safe than sorry.
That needs 3 30 amp 240v circuits added to my breaker box and 3 outlet boxes. I think I have the ability to do that with my existing circuit breaker panel, but it will likely cost another $500 to wire it all up.
Then I found a heat exchanger that will cool a barrel of boil in 10 minutes for about $300.
So:
Kettle - $800
Heating elements - $900
Heat exchanger - $300
Circuit breaker / wall outlets - $500
Misc hoses connectors - $500
Total - $2500
Then use a food safe 55-gallon drum as a fermenter. $100 or less each.