It's definitely efficient enough to operate when you are actually using equipment. We have a shut off switch to shut it down when not at our cabin. That way it is not slowly draining the battery while running nothing at all. It is not perfectly efficient, but the way I look at it is that we gained by charging at 24v we lose some of that in the conversion process. Also you pretty much have to go higher than a 12v battery bank at a certain system size. Specs on the unit say it is 85% efficient, and has a 0.2 (2/10ths) of an amp for a no-load current draw. The no-load draw is why we installed a properly rated shut off switch between the battery bank and the 24v to 12v converter.
As to the other question, the small charge controller would only handle your 300 watts at 24v. And yes you could run three 100 watt panels in series, I believe the max voltage to the charge controller will handle up to four 100 watt panels in series. (check the max input voltage in the manual to be sure).
If I were in your situation, I would probably get the 40amp charge controller, and an extra panel if you want, and wire them in series. Then just keep everything 12v until you want to replace the current 12v battery. At that point you could go to 24v. The larger charge controller would give you the flexibility to do whatever you find works best. Starting with that would give you time to learn your needs, and avoid purchasing another battery until you actually need it.
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