[AMB]

Yes total volume is 326 L using your measurements, but you don't want to overfill it. So say the total height of water is about 4 cm from the top (height 43 cm), then effective volume is about 298 Litres. Probably a few litres less than that, once you put in sand etc.

Heaps of info online, do heaps of reading. All depends on what you want, and how fanatical you are about it. You can go from very high maintenance (almost daily) to very low maintenance.
Here's a few tips:
The novelty wears off after 6 months, especially if you cant get the balance right. The tank gets dirty quickly, and you start losing fish. Always the main problem is having too many fish. They sh!t a lot. Then you feed them too much, and they sh!t even more. That's why the tank gets dirty quickly, too much faeces, and wasted food. Cant be helped when you have too many fish, the dominant ones eat everything and get fat, while the shy timid ones get very little and slowly starve.
Snails are nice, but don't expect them to keep the tank clean for you, same for bottom feeding "catfish", they will never compensate for an over crowded tank, (no matter what the sales person tells you). So people get bigger, expensive, complicated (and often noisy) filter systems thinking it will keep the tank clean. It does work, but it become very high maintenance, and you need to know a lot about it, some people just love it.
So the secret is to have just a few nice fish. In a tank that size, maybe just start with half a dozen at the most, until you get good at it. At least 50% of the beauty of a fish tank, is having healthy thriving water plants nicely arranged, so concentrate on getting the plants growing well, and the rest just follows. In a poor set up, the plants always struggle and look terrible (sickly looking and covered in algal growth etc).
Size of the pump and filter? All depends on how much noise you can handle, and how much fish faeces you need to deal with. A few healthy happy fish can last 5 -10 years and even longer, depending on the species, with just a modest air pump/filter setup. An overcrowded aquarium and you will be constantly replacing the dead ones, and constantly cleaning the tank.