Him, KdB and Mahrez are all 31. Walker is 32.
Just extend them all long enough to play together, they all probably have a couple more years for us at this level. I'd be happy with all of them here when they are around 34 (Walker 35). I assume at this point (3 years, they'll be too old to be a starter, perhaps a backup player, but defo no a starter).
During these years though we have to start getting there backups ready, don't buy the backup when they leave, you buy them before they leave to have a natural transition (B Silva -> D Silva).
Nunes seems a good long term target for Gundogan (or even just to replace Bernie if we lose him next season). But we will need to make a couple more signings + academy integrations over these 3 years.