Performance in most games is acceptable. It's really up to the developers though. Titles like Serious Sam 3 and Binding of Isaac are completely designed and built to run well on Linux-based systems and can easily outperform their Windows counterparts.
Then there's the developers that first built and optimized their games for Windows and hacked in Linux support later on, those aren't gonna run as great as on Windows but overall performance is still going to be good.
There's also a lot of terrible and unfinished port jobs which make you want to get your money back. Killing Floor and Dungeon Defenders are perfect examples of those, better stay away from them or wait until they've been patched (I wouldn't wait for Killing Floor though, don't think they're going to patch their game after all those years).
As for new games being released: Yes, they just keep coming! More and more developers and publishers are pushing their games to Linux and the catalog is expanding rapidly. I like to follow the http://gamingonlinux.com/ blog to keep up with new releases and performance reports.
If you've never built a PC before there's going to be a bit of a learning curve. The /r/buildapc community is definitely willing to help you with that though!
Installing SteamOS on your freshly built Steam Machine is going to be as easy as installing any OS. You can find instructions here: http://store.steampowered.com/steamos/buildyourown
Good luck!
They're pushing using the SDL 2 library (http://www.libsdl.org/) to make it cross-platform. I wonder how many of those 300 are AAA. And I hope performance improves a lot; the tiny bit I tried of metro last light under steamos seemed fine, but native "Steam Dig" runs horribly on my Phenom2x4/HD6870 SteamOS box for some reason. It looks fine but the framerate is very slow/choppy compared to under Win7.
Which packages are you using for the steam login? I'm using these and they bypass the desktop completely: https://launchpad.net/~mdeslaur/+archive/ubuntu/steamos/+packages
You then need to enable autologin in LightDM, I wrote down my Ubuntu Steam Box set up here if you wanna compare notes.
I've found Steam to be unresponsive in general when waking from sleep and just don't use it.
Just me being clever?
Also, Steam Machine Teardown.
You must be one of those people who don't realize Mini-ITX is a thing.