No, don't purchase the m3. It's very inaccurate, has a low spread but very low RoF and damage, It also doesn't stack damage.

But what I do recommend buying is the AA-12 (Auto Assault-12). You probably already heard of it, It's a full auto, high damage shotgun available for the price of 60k Gralats, the equivalent of 25$ USD
I'll post the stats here:

AA-12: $60000
Dual: No
Damage: 6
Spread: 0.125
Freeze: 0.3
ROF: 0.2
Reload Freeze: 0.25
Auto: Yes
Clip: 36
Bullets fired: 3
Stacked: No