18 years is long time for a cat. You gave him an awesome go at it and he loved you for it. He could have a had a shitty life, but you obviously loved him and saved him from that.
It's a rough one. My two cats are 7 now. I cant even imagine what I'd do without them. My condolences.
You can't replace a loyal pet. You can get others but they are different as children. Sorry for your loss. I've lost so many good friends now, it's hard to remember them all. It sucks.