Best sergeant gear to buy

[Gismo]

A good tactic is to grind as much as you can or with some friends, take IRK as an example. They get a lot of craftables and trade those for legendaries or something better.

but best sergeant gear would be

Grinder Helmet
Banded Mail
Studded (?) if not use Ornate
Heavy highlander boots

In terms of weapons
Aurora Blade / SGS
SKS