That estimate is quite a few years out of date!
As for the car, I think it's what you can expect for an older UK restoration that turns up for sale these days. Generally looks okay, but undoubtedly could get very expensive to bring up to the standard of what people expect in a restoration today. That's the kind of car that was knocking around in the early 2000s, when I got into Zs, for the 4-6k mark. Like my Fairlady Z, previous restoration had been done, but for every good bit of previous work, there was a corner cut somewhere else to sort out. The legacy of cars that were worth very little for much of the 80s and 90s and bodged or built to budgets. That's why your car is a gem Phil - no previous bodges to fix - just OEM rust!