Question: Will there be any upgrades, attachments, etc.? I assume DLC will come out.
There are attachments for the guns. Every gun has 4 or less attachment slots (top, bottom, magazine, muzzle).
And there will most likely be DLC (other than the Pre-Order ones).
Also, is there any estimate to the number of levels in the game, or playtime in Single Player?
There are 8 +2 maps (8 regular, 2 unlockables) in the game, which are played the same in Single-, Coop- and Mutliplayer.
Lastly, does "weapons" include gear/support items?
No.
There are 24 weapons you can shoot people with. In addition there are 5 x 10 (general ones + 10 for each class) abilities; these include turrets, mines, grenades and other handy stuff.