This guide really desperately needs updating to deal with the rust issue.
I must have looked at 20 GTVs over the last year or so and *every single one* has had rust somewhere.
So add checking of:
Floor pan, particularly around the D-shaped 'drain holes' at the rear edge. Even if you think it's OK, press the underseal here and listen to it crunch. I have yet to look at a single GTV up for sale that *didn't* have some evidence of corrosion living underneath the sealant and merrily spreading through the floorpan. I thought mine only had a little 50p-sized bit but on peeling back the underseal it was more extensive (car now booked in to get it cut out and a repair section welded in). Any stone chip digging into the underseal down here opens up other locations for it to start rusting at.
Under the plastic sill covers (sometimes showing up beyond their edges, underneath the car, by which time they're utterly knackered).
Behind the wheel arch covers (often showing as small bubbles on the arches, again by which time what's under the covers is much worse).
Between the bonnet hinge point and suspension turret there's a void filled with a sponge, the panel is mostly hidden by the wing, this can rust away and is close enough to the suspension turret to be an MOT fail with a strict tester.
Boot floor. Only seen two suffering here but both were well rotten with only the boot carpet providing any integrity at all.
Around the radio aerial.
Radiator support cross-member underneath the rad, though I wouldn't reject a car based on this as it's straightforward/fairly inexpensive to replace.
Probably more I've missed...