Yeah, good plan.
Most weapon drops are randomised, there are spots where you know there's likely to be a weapon but not what it'll be. A few are static (there's always a chrome shotgun in the van before the fence-climb on Dead Centre, for example).
The grenade launcher is rare, I can't recall anywhere where you're guaranteed to get one but there probably are a few spots. What it's actually best at is killing specials and tanks very very quickly (it'll kill any special in one (exact) hit). But it's obviously very hard to aim that well.
Shotguns are decent when playing advanced or below, so long as you don't run out of shots during a crescendo or something (easy to do, just miss a couple of shots then get mobbed). They are good for clearing out at close range though, yeah.
On expert and expert realism they're useless.
The magnum/deagle is also very good on anything below expert too. It's a one shot kill on mobs and will kill most specials in one headshot too. But I'd rather carry a melee weapon for when things get desperate.
The "best" weapons (I put it in quotes cos it's situational and personal preference matters) are the military sniper rifle and the AK. The former is just really powerful (but has a really slow reload) and very good for killing specials. The AK just has a nice balance of accuracy, power and reload speed. The hunting rifle is good too, similar to the military sniper rifle, not as powerful but faster reload.
*Ideally* you want to kill the zombies before they get anywhere near you but that's all about knowing where they'll spawn and finding nice angles and chokepoints, it comes from knowing the maps.
Melee weapons are good for when you're surrounded or trying to clear zombies off someone you're trying to get up. The "best" are the sword and the axe. The sword has the biggest killing arc and is pretty fast. The axe is a tad slower and has a shorter killing arc but does way more push-back. The machete is very fast but obviously has a very short arc. They're all decent. The tonfa is useless. The frying pan is kinda useless but funny. The golf club is great but only on a few maps.
The chainsaw is really good at attracting the horde to you so it's super useful in situations where... you want that. Like if two people are down, one of you can run around with the chainsaw drawing the horde off while the other person gets the two downed people up. It's also good for like holding a doorway or something.
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