Typical smoke signs:
Blue = Oil
White = Coolant
Black = Fuel
The blue @ startup is valve stem seals most likely. Fairly easy fix, but also pretty easy to live with (unless you crank the truck @ a car show or the track).
The white @ idle is most likely a blown head gasket or leaking intake gasket. Again, fairly easy fixes and something that can't/won't be tolerated long.
So, figure that the heads will most likely have to come off. Get a quote for head R&R from a shop as well as labor to change valve seals. Try to get this knocked off the price (or some $ at least) unless he's already discounting for it.
Other than that, sounds like a decent buy as most well-driven 12-13 yr old trucks won't be perfect.
Hood