I am a herd manager on a big dairy in south Australia. We milk holstein but I have worked with other breeds. Really it depends on what you want from the cows. Is the milk going raw to a processor or will it be processed on farm for cheese for example? It really does depend! Jersey and Guernsey have the highest milk solids followed by Ayreshire. I am not sure about shorthorn I am afraid. Generally Holstein have a lower milk solid, but have the potential to produce more milk if fed (and bred) correctly.
As far as choosing the right calves - whatever breed you go with- you want big,lively healthy looking calves. Shiny smooth coats, clean backsides (no diahhroea) clear eyes without discharge, and for dairy calves when you watch them walk from behind, make sure their back legs dont cross "model style" when they walk. They will be a pain to milk! The best advice I can really give you though is to speak with a local dairy farmer and if possible get him to go to a cattle market with you. There is nothing like local wisdom to help you pick the best animals.