Analog to MIDI is tricky because of the harmonics & overtones, and chords and multiple instruments are even trickier. You might look for a computer application first, just to get an idea of what's available and how well it works when you have enough processing power and good programming.
Note the example doesn't use a "standard" Arduino. It uses a more powerful variation.
From what I've seen on the forum, most people who try to make a guitar tuner are unsuccessful. 
It might be possible, but you might get latency (delay). That's not a problem (or rather it's a problem that can usually be worked-around) if you're "recording" and making a MIDI file but it's a problem if you are playing live.