Welcome Kesmit,
I will answer your second question first. Every single chorded song shown on Chordie can be transposed into whatever key you want it to be in with two button clicks so a search by key is not necessary. Look to the right of the screen for 'Transpose chords' click the button on the right , select how many semi-tones up or down you want then click on that, and you are now in your chosen key.
Your first question I find harder to understand so if I am barking up the wrong tree, I apologise. Many years ago when I was taking piano lessons (a skill sadly long since forgotten) I was taught from a music score where the number of sharps or flats signified the key used, but I gather from a friend that it is possible to play piano/keyboard from just using chords as we do with guitar but I would have thought that a pianist would be able to determine the key himself from the chords used. I do not understand how looking at the source chord chart helps as by looking at the chords used in the song in front of you should tell you what key it is in. For instance if the the predominant chords are Am, C, F and G7 the key is C. Also in C you will find D, Dm, Em and Bb commonly used too.
After writing that I have just remembered something!! Take a look at: http://www.chordie.com/forum/viewtopic.php?id=8500 the same question was posed and answered in the Theory section a while ago.
Perhaps someone with more understanding of piano/keyboard can help you better with this.
Roger
"Do, or do not; there is no try"