check this guide on how to reach phong nha from hue.
cheapest way to reach would be an overnight sleeper bus from Hue.
yeah its practical as long as you are not long (pun intended!). I mean taller than 6 feet!
the buses are fine but leg space is not enough for taller pax.
check here to find out how many days are enough in phong nha based on your budget and what kinda tours you like.
personally, I would stay minimum 2-3 days here