If you are unhappy and can get a refund, which I would be surprised about in all honesty as most used cars are 'sold as seen' and clutches are wear and tear parts, go for it. BUT bear in mind the same thing may happen on the next car you buy. Folk on here have had clutches replaced at 20k to well over 100k. As others have said it's all about how it's previous owner drove the car.
Alternatively if you get it fixed you know it's gonna be perfect.
Rule 1 of Zed ownership is completely ignore anything Nissan UK tell you and trust Clark Motorsport, ZMANALEX, Cougarstore, Horsham Developments, Kaizer etc
Labour to remove the box and clutch etc is about 3 hours disassembly and then 3 hours assembly. Thus 6 hours, at independent garage rates £40 an hour that's £240. New clutch about £300, new fly about £350 = £890.
I guess the quote you had for the clutch would include a genuine Nissan clutch from Nissan which they charge about £600 for. The garage being a chain will be running about £70 an hour labour thus the £1000+ price.
Unfortunately clutches wearing out is just one of those things, you never know when it will happen but it will at some point.