Well, i can buy windows with no problems, but probably to try something new, i would like to know if SteamOS...you know...is good enough?
Steam in full mode is more or less steamOS
The short answer is "Not really."
Even Linux aficionados find SteamOS disappointing to use. Just get Windows 10. OR, install a more mainstream flavor of Linux.
SteamOS isn't great and can be a huge hassle to install. If you want to save time and money, go with a regular Linux distribution and install Steam. I recommend Xubuntu.
EDIT: If it helps, I actually made a tiny SteamOS-like setup using Lubuntu.
It really depends on what games you want to play. The majority of big-name $60 AAA games (GTA V, The Witcher III, etc.) still don't run on Linux/SteamOS (for now). Plenty of indie games do, as well as every Valve game and some larger games (Civ 6...). Depending on the game, performance may not be as good as on Windows. But if most of your games run on it, you should be fine.
Adding onto this, a surprising number of smaller Steam games will run in Wine. So factor that in, too.
Witcher III is going to be linux very soon.
If you're going to go through the trouble of getting SteamOS onto a machine, you might as well throw Ubuntu on it. The game library is basically the same, games basically run the same, and it's not a big pain in the arse to use effectively compared to SteamOS.
SteamOS doesn't have directx yet, so you won't be able to play a lot of games on it.
It won't ever have DirectX really, that's quite Windows-specific. It has limited support via Wine (which basically translates Windows API calls into Linux calls) but that doesn't work for all games, not least of which because it doesn't support DirectX 11 yet (although they say they will by the end of 2017). The burden is mostly now on developers to either write their game using cross-platform APIs (OpenGL, Vulkan...) or port their engine to support those APIs. Most commercial licensed game engines (Unity, UE4, CryEngine) support Linux but many of the internal engines used by AAA devs (EA's Frostbite for instance) are Windows-only.
I actually don't even know why I said yet. You're right. Microsoft is very unlikely to release DirectX to the public as it is a large source of revenue for them.