Besides from the engine displacement, their differences are mostly seen inside. Steering wheel for 1.8G is a combination of wood and leather while the 1.6G is purely wrapped in leather. Same with the dashboard, 1.8G has faux wood panel surronding it. 1.6G doesn't have the all leather interior. Then there's the 2-DIN car stereo advantage. 1.6G also came out with AT trannys, but it doesn't sport the gate type but the more common AT pattern.
I would think that it is pegged at a cheaper price. Compared to 2nd hand lots. Do you know the owner? How his driving attitude and maintenance towards his/her vehicle?
Baka naman may hidden story yan kaya mababa ang price para mabenta agad? Just my thoughts, running wild again.
The final choice is still yours, it would be better if you scout other units as well. Then compared notes. If oyu find the unit still fresh in & out, go for it.
100K for additional cosmetic changes? I'd settle with a 1.6G.