Been playing a couple hours, I'm up to 'a quiet place' bar meeting mission.
Been doing a lot of trading, upgrading components.

The ship is pretty much at mk 2 everything. For weapons I salvaged a mk2 vindian laser turret and I'm pairing that with a mk2 mining laser turret, mk2 tach broadside and mk2 flak 2ndary.

So I've got 1/2 subsystems, I bought the first cheap one for mining, but I don't see that it does any good and I'm not mining much. I don't use ammo with my current weapons.

What subsystems should I buy? I've got about 300k saved up atm.

The mining scanner is only needed if you plan on doing any mining, as it increases ore yield. Mining is kinda a subgame, which can be used as a way to quickly get lots of cash if you stumble upon the right types of very rare ore. There are whole threads on how to do this.

Get the biggest cargo hold expansion you can afford, as that gives you flexibility on accepting missions or lugging around cargo till you locate a profitable trade hub. And of course you need a tractor beam for many story missions.

Subsystems that increase weapon range and shield recharge rate were always my default choices. The one that leads weapon aiming is good if you are using particle weapons vs fighters. But it depends on your playstyle. You can always recoup 100% of any costs by selling uninstalled systems, so there is no downside to trying a new combo and seeing if you like it, then changing your mind. So you can tailor your subsystems to your current ship strategy.