Replacing Gearbox, G60 flywheel and clutch?

[Chorn1.8t]

Hi all I’m replacing the gearbox on my 20vt Ibiza Cupra. I was going to replace be clutch with OEM but I’ve seen a few people recommending to fit the G60 flywheel and clutch whilst the gearbox is off. Could someone explain the benefits of this and if it would be worth doing or not. Could someone also suggest a good/ the correct gearbox oil to use?
Thanks in advance!