It's built on Symfony with a few cron jobs to process everything in the background and sendind the emails through Mandrill. I'll most surely add some kind of queuing mechanism (either with redis or RMQ) soon to handle things more gracefuly.
The design is a paid wrapbootstrap theme called Harp.
It's true yet it's going to be free as in "build it for yourself and maintain it all your life".
I tend to forget how I've built some tools I'm using and I sometimes spend hours re-understanding what I've done.
Regarding the price, services like feedly - disclaimer: I've been a paying subscriber in the past - cost $45 per year, and it's not been clear to me what value it was providing me (except some integrations with 3rd party tools maybe).
Anyways, I would certainly love to see an integration with RSS readers like the one mentioned above (which would also mark as read all the posts I received in a digest).
You definitely can, and in Muna's current form, I guess you even should :) The thing is, I have a lot of ideas that will make Muna unique and, hopefully, a lot more usefull, like offline and mobile reading support, content relevance ranking, etc.
In the meantime, here's a blogpost that explains the why and how of Muna : http://geoffrey.io/muna-your-personnal-paperboy.html