On Demand Hot Water, Nat Gas, Propane or Electric?

Hi,
I'm in the process of remodeling a small apartment over my garage and am trying to decide the best way to feed hot water to it.
I like the idea of instant hot water as for the most part the place will see little use, it has a show but for the most part is just a game room so no standby loss would be nice.
1)The gas people did agree to run a line to it with a few conditions but they charge a monthly fee on top of gas used. They implied if usage was too low they would make me pay for the line install. Not sure if they could actually in force that tho. Don't really want to deal with the agro of fighting them so have set this option to the side.
2)Electric sounds tempting and I have a 200A line in BUT it only feeds a 100A box and the heater I looked at takes 2 20A breakers. Could be worse as there are only about 4 circuits in it right now. It looks like it will hold 20 breakers so one thought is to just swap the main break or entire guts.
3)Another choice would be to get a propane unit, Bosch Aquastar for example. Problems here would be cost to buy tank and hassle of having to refill it.
So I guess my questions are; How would the running costs compare between options 2 and 3? How long would a 100 pound tank of propane run for? What would be the approx cost to swap the guts in a breaker box? Is there an better option to swapping the guts? Extra box? Any thoughts or experiences to favor either method.
A 3rd option I considered was to use a regular gas/propane water heater and install it such that it would also feed radiant floor heat via PEX tubing. One obvious question here is how hard is it to turn a gas water heater on/off say once or twice a week?
Thanks
Nick..
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Since your not going to use it very often I would opt for #3 propane. No 100 lb tanks get some of the 30's or 50's that RV's use. Heck a 20 would probably be enough for a couple of days. If you know your incoming water temp and the rate of rise. You should be able to calculate how long a 20 pounder would last.
Option 2 convert some other equipment in your home to gas and use NG.
You do not swap electrical guts out very often. Panels are made a certain size with a certain size bus. Trying to make changes is asking for a lot of trouble. Field modifications are usually a no-no for insurance companies.
Look for a gas water heater that has electric ignition. A 7 day time clock solves your problem.
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