While 6,000mAh doesn't sound like much, remember the Tab S2 had a 5,870mAh and it's performance was quite good, so battery life should be better with the S3 (hopefully at least)
That's Octa-Core. It comes from the same root as octagonal and means eight. In this case it means that in one chip there are eight core processors.
Sorry, but having owned or owning a Surface Pro 4, various Samsing tablets, iPads and such the best way to achieve ergonomic and productivity perfection is to buy a Logitech K800/810 Bluetooth keyboard. Not only does it have excellent full travel keys and the ability to hook up three different devices, but it's backlit and the rechargeable batteries last ages.
You can position it wherever you want. On the desk, with tablet on your lap. Keyboard on your lap, tablet on the table. Both on the table, etc.
I will never go back to having a keyboard whose position is forced by it's physical connection to the display.
Samsung are simply copying Apple's keyboard / pogo pin concept. Being iPad sales are down, it has not proven to be popular.
Take my advice, buy the tablet, but not the keyboard, and get a seperate Bluetooth one.