the chassis box section (rear) check the fixing points for the anti roll bar and diff then follow the chassis to the front.
the sills inner and outer (if it looks solid from the outside don't trust it make sure you get the insides inspected with a pin camera especially if the dealers asking 20k plus and if he is be very careful and do your homework)
the inner rear roof sill and outer lip (best to remove the lining at the rear and inspect)
the rear boot inner both sides where the turrets meets the boot floor (remove all the linings and inspect)
open the rear doors and pull of the black felt strip and inspect the seam
pop the bonnet and look down both suspension posts rot starts here and eats into the supports
under the front arch guards remove them and check the suspension legs (tap gently with a small rubber hammer) also a major hot spot is the front panel that joins to the front chassis beam (also look at this from under the bonnet as its double skin and moisture builds up between them and rots from the inside out)
the windscreen surround is a problem so make sure you waxoyl it all the way round (bottom can be accessed from under the bonnet)
floorpan and jacking points!!!! check photos the jacking mounts failed as soon as the car was jacked up
hope these tips help
ps strip it and zinc dip! if the shell is good its worth it check my rally build photos ffi this shell had the advantage of abarth chassis plate kit and it was rot free before dipping
