First, drinking/peeing a lot can mean so many different things as can one elevated liver enzyme. Getting a Cushings diagnosis can be scary but the key is educating yourself and making certain that Barney is being diagnosed accurately by a vet who is well versed in all the aspects of Cushings. I would strongly recommend that your vet refer you to an internist who specializes in Cushings diagnosis/treatment. Make certain that you obtain copies of all of Barney's test results. I would also recommend that you join and start reading on this website:
Canine Cushings.
The members are unbelieveably knowledgeable and supportive and can offer immediate suggestions as to testing recommendations, interpretations, etc. My Maggie has both atypical and typical Cushings (atypical being increased hormones other than cortisol; typical is increased cortisol). Testing and loading can be expensive but, once you get over that hump, treatment costs drop significantly. If Barney gets a definitive Cushings diagnosis, the Cushings forum is a good place to help you decide whether to proceed with treatment by weighing all the options.