Hi and welcome.
I would try searching for some leather seats now before you buy the car . I'm not sure how easily available used leather seats are for the e89 . There are plenty of leather trimmers who would do the upgrade on the cloth seats but I would think that you would have to pay over a thousand pounds especially if you go for Nappa leather and the array of designs.
Good luck and I'm sure there will be more advice to follow
Depends on where you use. We had a R32 Skyline re trimmed in nappa front and rear for £1200 which was a excellent price. You can find leather re trimmers near you online and they would happily give you a quote bide ally you'd also want heated seats installed. They are a bit chilly in winter.