90 spec definitely came in in 1989.
If this isn't a 90 spec, then the owner went to a *LOT* of trouble to make it one, i.e.
-added big bumpers, and correct door rub strips, and side skirts
-fitted full rainbow interior, including correct door cards
-fitted smoked rear lights
-converted to CE2
-added larger airbox and 50mm inlet manifold
6-speakers weren't standard until towards the end of the production run, and for some reason all the G-reg (90 spec) valvers I've seen don't have electric windows!
In the words of Rafa... if it is white, and in a bottle, then it is milk!
The car looks good - I'm guessing it is a little too pricey?