You rank by both buying engrams and by having the ones you found identified. I hit rank 3 yesterday and got 3 blue items from them as reward. The blue engram ended up being a lvl 9 green shotgun... there were also some crafting materials.
I tried all the guns and I like autorifles the most Just spray and laugh Autorifle / Sniper / Machine gun is the combination I like most. Autorifles for typical enemies, snipers for enemy snipers and distant hard hitters and machine gun for downing elites or stronger types (Wizards, Hollow Knights, etc). In the special slot, the fusion rifle is also cool for wrecking things to pieces. I do not like the shotgun at all. Hand cannons are nice for precision kills, but they're not as fun as autorifles and take longer to kill things for me.