This guide covers few mobile suits that you can get early on is good for the entire or at least a majority of the game.
Reliable Suits Overview
If you have the Pre Orders, Knight Gundam and Commander Gundam can keep you stable for a while.
SD Commander Gundam is a good sharpshooter just play range only and a lot of dodge. Make sure your partners are melee and either all-rounder or also Sharpshooter.
If not… Barbatos is good enough, Gouf custom in second chapter is also very stable too as an infighter.
Mind you may have a few problems with enemies in the air but if you want to use a Sharpshooter, command or ground GM can offset those issues heavily. just keep in mind your Health pool wont be as large.
The Ez8 to be a better Sharpshooter than most ranged specific gundams, its melle also hits harder because its an all rounder, the only complaint is its ultimate is prone to have a team mate knock the enemy out of its limited area of attack once the first strike lands, but it does decent damage if it all connects.
You’ll always need to keep any suit upgraded to keep up with mission difficulty.
In general use whatever you like, but one good idea to keep in mind is to keep a sharpshooter type MS that is all physical. The reason is because when you run up against bosses with I field generator, beam shooting attacks are completely nullified until you deplete the I field bar, so imo, you might as well just use physical ammo instead.