You certainly an use the 15 gallon keg, but don't expect to be doing 10 gallon batches or anything with 120V. I've used a few different systems with 120V/1650W elements. Those do OK for 5 gallon batches, and with the freedom that electric brewing gives you with a controller (not having to babysit your brew continually, being able to brew indoors- with good ventilation), it's probably worth it. What's even better though is a 120V/2250W element. This requires a 20A/120V service (outlet has one vertical plug blade, one horizontal blade), but performs phenomenally better that the 1650W elements.
Depending on what you want to do with your 15 gallon keggle, you could go the RIMS route with a 2250W element (or Blichmann RIMS Rocket 2000W) and do eBIAB with a pump to circulate things, and have a second 2250W element (BrewHardware has good affordable ones) that's in the keggle that you switch over to when you pull the BIAB bag out and are ready to boil. Blichmann also has a 2250W/120V Boil Coil that would probably fit in there (guessing, check your measurements), but will be pricey-er.
As for a controller, Blichmann's new BrewCommander is pretty spectacular. I just finished reviewing one and was super-impressed. Their old controller was over-priced and under-featured. This new one has a ton of great features and comes in cheaper than much simpler/lesser controllers.