It would be 100% efficient, just like any resistive heater. All the electricity put into it ends up as heat including any noise from pumps and lighting. Of course it would only be a few hundred watts unless you're talking a big well lit reef. You can't really quantify the value of it being nice to look at, but surely that would make it better than something like an oil column heater of the same output.
You can also rent bottles, then you don't have to worry about any safety testing. I don't know what options on sizes there are though. My father in law rented a massive 3 foot tall one for $10/month I think.
I do think though that a 1200X600X600 is a nice size tank. It's not particularly big, but it's big enough to give you a good amount of space to play while just starting to edge into big enough to start being more inconvenient to maintain. . You can't really get your hands down into the back bottom of the tank(Or at least I can't and I'd say I'm about average), but the rest isn't bad. Smaller you start sacrifice flexibility, bigger starts to be a bit more of a mission to plant, aquascape etc
I'd make a guess the 450 Pippin mentions is similar dimensions? There are 1800X600X600 which are basically the same, just use up more wall space. They'll be good too.
Jack dempseys, discus, firemouths, festivums, Surinamensis/altifrons, green terrors, red terrors, brasiliensis, jaguars, chocolates, lots of others I can't think of at the moment.
SA, definitely. Much nicer size, not usually as aggressive as Africans. Don't prefer really high PH which limits any plants or other fish you can have in the tank. SA have more variety of sizes and body shapes. Africans are all the same fish with a different paintjob.
Optimum tank size is always the biggest you can get.