It seems like cancer, kidney problems, diabetes and hyperthyroidism are pretty common in older cats.
We have the cancer and the hyperthyroidism, and his kidneys were not in great shape when we last had them checked. However, that was when he was also very sick and dehydrated so I'm not sure what they are like now.
At the moment he has not shown any nausea. He did have his thyroid tablet compounded into a palatable kibble, but since he has been more picky with his food I have had to go back to putting a tablet down his throat.
He doesn't mind too much. Probably because he gets a chunk of butter to lick afterwards.
The vet said feed him whatever he will eat. At the moment that seems to be various raw meats and ... fancy feast. I'm sure if I got a tin of whiskas he would be all over that as well haha