Hi Numazu,
I was in Yangon a couple of months ago. Basically just stayed in Yangon and didn't venture out.
Should just pick up a data sim card at the airpot. Supposedly MPT has the best coverage. Oredoo and Telenor are the other two.
For wifi, I could just pop into Sule Shangrila or The Strand, buy a cup of coffee for 4USD and get 1 hour of wifi. They are that strict/stingy with wifi!!
Here's an amazing trip report I used - http://pohtecktoes.com/myanmar-itinerary-for-first-timers/
Enjoy yourself in Myanmar!