So I've decided it's time to put my old dog to sleep, can I have some advice please?

I'm so sorry for you, this is the hardest part of being a pet owner. Your dog will not mind what you do, I have never asked for my dogs to be cremated or asked for their body, I prefer to get a nice photo of them when they were healthy and happy so I can enjoy thinking of them. Ashes are just another thing to dust, they are not your dog anymore.