Which gun do i get?

Weapons that you have not leveled to rank 30 aren't "done" yet, so don't throw it out. You get experience from finishing things (getting them to 30). Also don't forget to mod your weapons and warframes if you are new.

Any weapon can easily get you through basically all content. pick a weapon you think is interesting or cool, and the rest is a matter of the mods you use on it.

You're probably a bit too weak.

You could make a braton rank 23 work, but you'd need good mods (which you probably don't have yet).

My personal recommendation is maybe do other content, hit MR4 and get yourself a Hek.

The Vectis is a good weapon at a low rank, does high damage and can help against Eximus that give you trouble. Imo sticking with either rifles or shotguns will help, since you can pour more effort into one category’s mods and get stronger overall. Otherwise it’ll take twice as long to get both sets of mods leveled up to do content

also i am only mr 2

I got stuck on that same mission my first time through. The thing the game doesn't explain to you is that the open worlds aren't really meant to be played until later in the game. There's some content you can do in them but in general you're going to struggle with every mission on them because you're not strong enough yet. I'd recommend working your way through the star chart and come back to the open worlds when you've got better guns/frames and mods

At MR2 your options are limited. And the Vox Solaris excavator defense is probably easier to do with a frame that has some crowd control or ability to defend the excavator. What frame are you using?

Build yourself a kitgun, they rule at low MR when all the good guns are locked.

Vermisplicer primary can punch through loads of content as one of the very few AoE weapons available at low MR, but requires a few trips to Deimos for the parts.

Catchmoon secondary has loads of punchthrough for nailing targets in a line.

Rattleguts Primary is a direct replacement for your braton.