I'd never pay £5,100 for a car that's been so messed about with - especially as you only have the owner's word for it when it comes to quality of parts used and care taken. I'd sooner part ways with that sort of cash for a mint original G40 that's only ever been dealer serviced and kept in one of those car bubble things all its life.
But as others have said I think they've now reached the stage where they're in the disposable car price bracket, which is a shame I think. Just like you can pick up an BMW 7 series for £2,000. But people forget that it's not just buying and insuring a car like that, it's running it.
There are so many badly bodged examples around, and enough clueless potential owners who'll heavily modify them before seeing how well they've been looked after it's hardly surprising there are lots of broken ones about now.
I remember when I first got my car, G40s were pricey to buy and pricey to run. There wasn't much in the way of tuning parts available without importing from Germany, and I remember friends being surprised I could afford a Mk3, let alone a GT, as my first car. Nowadays people are surprised when a GT is priced at triple figures.
Personally, as I've said before, when you're talking disposable car money, doubling the price for a good one is a sound investment.