Welcome Sir to the HT world. There are lot of articles in this forum to help you. Nevertheless, some thoughts:
1) for the size of your room, you should eventually look at 7.2.2 or 7.2.4
2) you should look at min 9 channel AVR as you would not want to upgrade AVR when you decide to add more speakers
3) 5.1.2 is a good start, but to get a true immersive experience 7.2.2 or 7.2.4 should be on your mind
4) out of 3L, 1-1.3 will be consumed by the AVR itself. from the balance, getting a good 5.1.2 for the room size might be little difficult.
5) For the room size, towers would be highly suggested