I would probably get the turret for the extra hp first, then the tracks, engine, and last gun. If I remember correctly the last gun is just a reload time buff (not 100% sure and can't check atm, since at work). Also I dont think you need the WAR, as I rarely get my ammo rack damaged, much less I've never been destroyed via ammo rack in the m103. Extra view range, or faster aim time would probably be a better choice for your third slot.
The last gun is actually a huge upgrade over the 2nd gun. It's the tier 10 gun the 110E5 uses and has like 20 extra pen or something like that.
I don't have much time to play this weekend (only have 2 games in it so far) but I'll play a few more games. If I don't get ammo racked in the next 10-20 games I'll take off the WAR and toss on a VSTAB.