I'd say just a good barrel and run it off an LP output tank.
Now, if you do upgrade other things down the road, there are products you'll be investing in and products you'll be wasting money on.
BOSS and Lurker have the game down. TechT, NDZ, Rufus Dawg all make sub-par stuff compared to them.
Well, there are a ton. The prime and most budget friendly (coincidentally) option is the Lurker Eigenbarrel.
I'd say after an Axe, my 2 main digs would be a .684 eigenbarrel (and a .678 if you're willing) and the BOSS infamous/XSV bolt or the Lurker Eigenbolt.
Other things, like say your foregrip feels awkward, Nummech has a magnificent and comfortable grip extender which can also be angled, I think, in the near future.
Triggers? I've tried an element. I'd suggest his before I'd even suggest a Violent, although the Violents are godly as well as the Acid Customs triggers. I have plenty by both manufacturers and can't say enough good about them.