timed ppsh is actualy better as it doesnt need repairing and is thus completely free to use
and yes from shop bizon is the best choice, pretty much equally good as ppsh, but it costs money, i personally only use it oveer using ppsh, because the muzzleflash on ppsh is killing my eyes
nonetheless the best is of course bizon arachne when it comes to co-op as its simply insanely improved bizon, but its a capsule and thus might be slightly hard to get
another good coop weapon can be spas15, if you have magazine upgrade b (adds +20 ammo to any weapon in coop) making the clip size a whopping 26 (or even 27 if you have shotgun skill that adds +1 ammo), this weapon is parrticularly usefull as it lets you solo BG extreme if you put trigger 1 on it (better accuracy and ROF) with mag B
not really any other goof co-op weapons, the hishest ammo count SMGs work best <and are your only choice if you dont have maazine upgrade B, othervise even other weapons can be decent such as sa58 para (with rifleman perk +1 ammo and mag +20 makes it 46 clip size with sick damage and ROF, othervise you are reloading all the time and its useless)
nonetheless pointman is still better due to all the running skills that also help you avoid damage