As long as it's properly fixed before purchased, I don't see that as an issue, it's pretty common on 350z's. As @Stutopia said though, personally, I'd be wanting to look under the car to see to what extent it's actually rusted, check MOT history and see if it's had any deeper issues in the past.
The W Brace and Member Stay Brackets are very common rust points too, they're considered 'structural' so it would fail an MOT if these were rusted. You're looking at around £600 for the parts and then whatever fitment would cost you if you'd be taking it somewhere (it's pretty easy to do yourself, just a few bolts).
At £6k, it should be in pretty good condition, you can pick them up for around 4k with similar issues you're describing so providing the arches are fixed well - £6k is a fair price.
Good luck with your purchase!