WELCOME TO YORKIETALK
You are to be commended for taking the time to research the breed before purchasing. That way you will be thoroughly prepared for your new little one. My 6 Yorkies range in size from a chunky 8 lbs. to 3.5 lbs. They are all healthy and happy. If you want a small Yorkie, there are many out there that are healthy and well adjusted. Just be cautious and stay away from anyone who advertised "tiny" cause quite often they charge extra for a Yorkie being extra small. Be sure that you have a good solid contract that gives you 24 to 48 hours to do a checkup and have a thorough exam by a vet well versed with Yorkies as soon as possible within the time specified in your contract.
I would recommend that you visit as many breeders as possible and look at as many pups as you can before making a decision. Look at the facilities, the parents, and the breeder. If you have a bad feeling, then keep looking there was a reason you didn't feel comfortable.
As far as food goes, there are several superpremium dog foods on the market. I prefer Canidae All Life Stages. But there are others that are excellent as well - Life's Abundance and Innova are also excellent. Be a label reader and do research on the food you select to be sure you are providing the best possible nutrition.
Be sure to select several interactive toys that will keep your Yorkie's mind active. This is a very intelligent breed and they get bored easily. A bored Yorkie = Trouble
Good luck in your endeavors -- I will look forward to your future posts and I can't wait to see your lucky little furbutt.