That mossberg 500 is a decent gun especialy in a combo pack like that - you have the right idea when it comes to shooting slugs with the Smoothe barrel.
Ya may also check out a Remington 870 - Best Pump action shot gun IMO you can get for the price im sure many will agree.
If you want to go even less on a shotgun to budget in a rifle as well check out a Mossberg Model 88 nearly identical (from what i can tell) to the 500 but more inexpensive.
My buddy aslo just started hunting this year as birds are coming up first he went out and bought a Pardner pump shotgun from walmart 205 out the door (heavy gun but decent to start) a 15 dollar bird vest and a cleaning kit from winchestor for 10 bucks...
We are going grouse hunting tomorrow :P thats all it took to get him in the filed for birds. (Aside from ammo).
Good luck and welcome to the Life.