Greetings! So I am planning to craft all sets of witcher gear and upgrade them. Right now I have a couple of swords and armor pieces that, if dismantled, give such components, as glowing ore ingot, dark steal ingot and plates, cured draconid leather etc.
So here is the question: should I dismantle them? Or will I be able to buy all the needed components from merchants as I lvl up? Right now I am lvl 12 and they do not have such ingredients.
My thanks!

Just sell everything you see.

Don't keep anything but the rarest drops from monsters, silver ingots, meteorite silver ingots, dark steel ingots/ore, but even these you can easily buy later on as coin is abundant from pretty much level 5 onwards.

Before going to last set of story missions, somewhere around 180+ hours and all question marks cleared, I didn't have enough materials to I craft any full mastercrafted set. Materials I lacked were dimeritium in various forms (plates, ore, ingots), glowing ore ingots (needed for dimeritium), cured draconid leather, and silk. Though I was able to run around craftsman, and buy everything I needed. After finishing crafting all three sets, I sold remaining crafting components, that freed around 70 units of weight and net me hefty 35K gold.
So it's your choice whether to dismantle things or use them. Generally there is little point to sell rare and relics, but on the other hand, keeping all components is also not exactly practical.