Hi i need to know how much my dog should weight in order to be happy and healthy?

the seller told me that he is a sheltland sheepdog and beagle mix, but i think that his facts were wrong because he looks more of a beagle lab mix with maybe a bit of pitbull and look nothing like a sheltie he has no sheltie traits watso ever and we cut off his meat diet after deadly dog foods started arising all over the news and its been a while since all that but he looks really weak and skinny to what he used to be on a meat diet and i was wondering how much should he weight in order to obtain his mive physique he used to have he's 30 pounds now and 18 inchs tall, all black except a little white blaze on his forhead and a white chest like pitbulls and his paws also look like a mix of black and white like pitbulls mostly have em white his are like greyish as if black and white mixed up, i know he has to have a beagle in him due to his litter mates looked just like beagles but he seems like a lab due to tail and ears plus the body except the height and the muzzle,ruff like pitbull