I would be concerned that the AWD system might have a fault, and some unscrupulous person has inserted the FWD fuse to mask that fault. Could even be a previous owner. I would either walk away from the car completely, or at least get that FWD fuse taken out and go for a LONG test drive. Also have your mechanic re-check the car at his own expense as he really should have noticed the warning light!
If the FWD fuse has not been inserted then you will want to get some proper diagnostic work done, or risk buying a 2WD Forester!
MY2005 2.0 XT, 4EAT
MY2003 Seat Leon Cupra, 6MT