Which Outfits should I buy?

There is an armor that you can't buy that is very strong on medium or lower difficulty levels (it is intentionally nerfed on higher levels). For me this armor did obsolete all other armors and I used it exclusively once I got it. You can get this armor once you progress further in the main quest, it is roughly 2 big quests from where you are now.

Spoiler for the exact quests you need to do:

You get this armor from the Ancient Armory side quest, and the last two power cells are in the Grave Hoard and the GAIA Prime facility.

Spoiler for what the armor provides:

The armor provides a shield that absorbs damage and refreshed automatically. It provides a lot of protection compared to the non-DLC armor with crappy mods, and also is a big boost to convenience as you need far less healing and herb gathering. It is significantly weaker on higher difficulties where it'll absorb only one hit usually.

If you use stealth, the stealth armor is still your best bet. When I played the game first, falling back to stealth was typically the way I resolved situations I failed at with brute force. For combat I prefer melee protection, for me when a machine got close enough to hit me that was generally the most dangerous situation. I found elemental damage protection too situational, so in the end I used the stealth armor with melee damage mods.

My impression is that unless you farm high-level mods and are in new game+ where you get an additional slot, the differences in armor are not that big until you get the armor I mentioned above.

The DLC added some more powerful weapons and armor, and a few very powerful non-random mods. It also added some enemies that are far more difficult than the ones in the main game, but you can do quite a few side quests in the DLC without hitting too hard of a difficulty barrier. In the DLC there are some very strong fire-based machines (and some ice-based as well), for these armor against fire is quite useful.

It depends on the area you are going to face.
I bought and invested in Protector armor, which works well against melee attacks and it's useful in different cases. I found long range protections quite useless.

Surely it's good to have fire-protecting and ice-protecting armors in late game. Elemental defences are situational, when facing specific machines they are a must-have in harder difficulties. As a side note, I prefer to maximize a singular element, instead of using more general mods. It's better to do something at best.

As @Mad Scientist is asking, the DLC area is strongly based on ice element, but fire-based machines are pretty common, too.

Really, I would say the armors are pretty balanced with eachother, depending on what is the best type of armor you can buy at that point in the game.

So after that the answer of 'what outfit should you buy' is pretty much decided by how you want to play and what are you facing.

If I remember right there are basically 2 types. Stealth (To snipe ennemies from the shadows), Combat (More defense, so you can fight with ennemies easier). The combat armors are also split in a bunch of different resistances. So you can go stealth (which I find works well with human ennemies) and resistance for whatever element you find hardest to deal with or that section of the game is heavy in.

2 pieces of armor are not in the 'balanced' rule.

First, the ancient armory armor, which needs you to find 5 power cells that are hidden on the map. That armor has pretty good stats (but more general based), and gives a recharging forcefield that will take the damage before your health.

Then there is an armor you get for completing the DLC that also has good stats and gives you a small life regen overtime. But the DLC is really balanced so you do it after the main game. Its gonna be VERY hard or even impossible before finishing the main game.

Both those armors are very late game. So yeah, don't bother with them until you are very far or done with the story.