He could very well be mixed. He is awfully big and should have a longer coat by now if closer to the yorkshire terrier standard. And since you did not get to see where he came from - I would not be going down that dirt road forever either to look at a pup (he very well could have come from a Puppymill). And with his price so cheap. I would get him neutered. If you are going to breed you have to make sure they are great representation of the breed. Have you checked out the Yorkshire Terrier Standard yet? You probably should if you have not -
Yorkshire Terrier Club of America There is a wealth of imformation there and will really help you learn a lot about yorkies. Also there are some great books out there about yorkshire terrier, for people wanting to breed and has more information about the breed - more than the pet information.
Also breeding is not cheap. Most people loose money, so hope you have extra money you can put into the breed and not expect to get any of it back. My last c-section (with my 6 pound female) was 809.90. Then you have shots, puppy care, food, pee pads, equipment, not to mention the tests to be done on parents before breeding, testing the pups before they go to their new homes, etc... To do it correctly you cannot do shortcuts and so is very expensive. If you work - also expect to take time away from work to whelp and care for a litter, a week off if mom has a c-section. So you have to make sure too you can take this time off. There is a lot to breeding than just mentioned here, but some information to think about.