Hi, I am trying to build an artificer (from the eberron book, not from UA) from lvl 1, thinking to play him all the road to 20. Probably gonna use the buy point system.
The main idea is a mad scientist gunner who loves to customize his bullets (specially with debuffs), mainly focussed in battle humanoids more than monsters.
As his personality, someone whose main motivation is to discover, learn and recreate any kind of technology, his dream in life is to create his very own Legendary Weapon.

As for the help, mostly want to know posible subraces, subclass and multiclass recomendations (or feats that may give similar features, like a fighting style) i was thinking of a researcher background or even spy (as I want him to be oriented to battle other people than monsters), some feature combos that I should take in account, any ideas are welcome.

Might I suggest a Mark of Scribing Gnome, also from Eberron? The race gets a +2 to Int and +1 to DEX, which pairs well with a gunner, which will need that INT for most things but DEX for ranged weaponry. The race allows you to roll a D4 and add it to the total for any History check you make, which should pair well with your character's backstory of seeking knowledge. The race also gets some additional spells... Message as a cantrip, and once-per-day Comprehend Languages and Magic Mouth. There are more spells that get added to your class's spell list as well, which helps to augment the fact that Artificers have a fairly small spell list to pull from.

Although Dragon Marks aren't totally balanced, so some DMs won't allow them. If not that, I would go Rock Gnome, which is already a kind of a proto-artificer (they get double proficiency on History checks, but only on specific topics), plus their tinkering allows them to build simple constructs.

Subclass could go two ways... obviously artillerist is the natural choice since it's, by default, a ranged-weapon focused class. But I actually prefer Battlesmith... it's a bit more straightforward, plus instead of a specialized gun you can summon temporarily as an action, you get a permanent ally in your Steel Defender. If you do decide to go Gnome the Steel Defender is actually a size larger than you so you'd be able to ride it as a mount. Plus if you put an infusion on your gun as a Battlesmith, you can attack using your INT modifier instead of your DEX, so it allows you to focus more on that stat. Artillerist kind of works better if you're attacking with Cantrips and spells but just flavoring it as using a firearm (like instead of "casting" firebolt, you just have a gun that spits fire), but if you actually want to attack with a traditional firearm, I'd recommend Battlesmith.

I went the reflavoring route with mine. Fire Bolt is Molotov Shot. Frostbite is Icicle Shot. Catapult is Cannonball.

I wanted a grizzled Dwarf veteran with a big gun and lots of practical battlefield skills. I also utilized Eberron dragonmarks to get the only subrace of Dwarf with an INT bonus and flavor his lowish STR and high INT as being old and having learned a lot throughout his life. He comes off much more like a greasy mechanic than a wizard that makes stuff.