SNIP

On one hand, i can get some slabs at a "friendly" price, but what i'm not sure
if it's a good idea given the uncontrolled drying post-cut.

I can buy kiln dried wood from a lumbyard instead, more expensive...

END SNIP

After the original year per inch or so It has nothing to do with time, and if
you buy kiln -dried wood and then put the bench in a humid area, you will have
wood movement the opposite way.  Wood is just a hygroscopic bundle of straws.
It constantly shifts its moisture content to match the surrounding air.  The
"rule of thumb" takes the wood from hyper saturation in its green state down to
equilibrium with the local air in which it is being dried.  In most places with
regular rain, that will be about 15% to 18% MC.  There are tables that will show
the MC equilibrium for each percentage level of relative humidity.  If you start
with wet wood and it dries in place, it will move and crack.  If you start with
dry wood and it gains moisture, it will break joints and warp -- and sometimes
crack.  Kiln drying just takes it to a given MC at the time it comes out of the
kiln.  Once in the open air, it starts to readjust.  Unless you keep dried wood
in a dry environment, it will go back to a higher level.

SO, ideally, you will build with wood that is within a couple of percent up or
down of the equilibrium MC for the relative humidity in which the bench will
then be kept.

OR, you can work up a design that will allow for the certainty of dramatic
shrinkage from newly cut-"green" wood.  BTW, even when we cut standing snags
that have not been actively pulling water into themselves for years, they have
very high moisture -- 30%-40%, even 50% MC.

J