As I said, I'm pretty new myself... so please take my recommendations with a grain of salt.

FN49 will become powerful in the future, so investing in her isn't bad, but I cannot speak for her current performance since I have not used her.

My personal (for the time being) general squad is Wa2k, Nagant, M14, Gr Mk23, and PPK, which isn't really an optimal mix but it's what I have for the time being.

So, there are three kinds of rifles. Bamboo (AKA Boss Nukes), RoF, and FP. Wa2k is RoF while M14 is FP (which is why the setup I listed is not optimal). The handguns you bring depends on the rifles you bring, and vice versa. So, basically I just look at the buff tiles (because honestly, I don't have a wide selection of handguns myself). RoF can be a bit tricky in general because it is capped, so there can be too much of a good thing. This is a better guide than I can give on how the formations should look (worth remembering, handgun buffs get stronger as they link up). As for budget handguns, Astra, Makarov, Nagant, PPK, Viking etc... basically, look at what they give, and try to buff your rifles as high as they can based on what the rifle self buffs to complement it. Again, take what I say with a grain of salt, because I am pretty new myself.

Oh, and one more thing. You will probably want to level your ARSMG squad to 70 or so if you plan on finishing the collab. Here's a guide to help speed that up. Learning to kite the final boss's attacks is essential as well