I'd say the Gage Sniper Pack is a good start. The Platypus we get in the base game is okay for Very Hard and below, but you may find it lacking in punch once you hit Overkill. On Death Wish you might as well leave it at home.
The Shotgun Pack offers a lot of interesting ammo mods, so it's worth a look.
If you're looking for a pretty solid compact primary for stealth, I'd also recommend getting the Golden Grin Casino pack. $6.99 is pretty steep if you're a casual player, but the Cavity 9mm is a neat weapon to have. The heist is fun to play if you can rally some players to help out. The rest is either not that interesting or requires some time to get used to, and may not fit your overall playstyle all that well, so it's your prerogative.