Welcome, and congrats on choosing a Yorkie, they can be a world of fun
I would recommend spending just a little more time researching the breed before you do choose to take a little yorkie bundle home, as no two yorkies are the same. They are very unique and highly intelligent little dogs and you need to be prepared for the training obstacles that this breed will present you with. So while you adore your friends yorkie, trust me, another yorkie could make you think twice about them. Take a look through the training section so you can see what you're in for with training
As far as training goes, I tend to think that the dogs they list as the most "intelligent" breeds, like the poodle, german sheppherd, etc. are reactor dogs, they give you a reaction when you give them a command...Terriers (esp. Yorkies) are listed much lower on the intelligence scale, in my opinion, because they are thinkers/analyzers. Every little thing you do they will try to figure out, so standard training methods do not work with them, as you will get quite a few blank stares :P.
Potty training, it is their number one training issue, probably because we as humans like our houses to be fresh and clean. You must, must, must have patience when potty training yorkies. They will be perfect one week, then regress the next, they will test your patience and you must not ever lose your temper with them. They will not forget it.
The above posters listed some pretty good information for when it comes to choosing your breeder, and I don't really have too much to add, other than if they are trying to push you to buying their puppy really think about walking away. Breeders should want you to come to them and show that you are really ready, and willing, to take on the responsibility of a puppy. They wont just give their puppies to anyone.
And, is this the right time for you to be investing time in a puppy? I don't know your situation, but I've been through divorces with friends and the ammount of stress they were under, I couldn't imagine them taking on the stress of a new puppy on top of it all. Of course you may have a totally different situation going but I did want to present it just in case, and for others considering a new puppy.
Other than that, good luck with your search just in case this breeder shows you some red-flags and you wind up having to walk away. If you are patient you will find your "perfect" puppy