Several good vendors on the forum here and a couple vendors on ebay that have served people well, I have had excellent service with buying from forum members and also ebay vendors.
I do believe that some of the china girls are put together better than others, there are so many factories over there that churn these things out and some of them use lesser quality bearings and the castings are not as good as some of the others, but I still stick to what I've experienced personally and seen here on the forum and say, all of them are a crap shoot as to whether or not you're getting one that will be a good runner and fairly trouble free out of the box when properly installed and tuned up.
Some people that are very skilled are building some of these engines ground up and making many needed upgrades and then selling them at a premium price $500-$800, personally I wouldn't spend that kind of money on one of these engines but it is an option that seems to be good for those who want to invest that much in a few extra MPH and the look of the custom cyl. head and other goodies which I also like.
do plenty of reading here of the forum and you can make an informed decision on which way may be best for you to go, I have voiced my opinion somewhat reluctantly many times about certain vendors and the positive results I've had with their products, I kinda hate to say much because I believe there are others here that would give me the same positive opinions but I just have not done business with them yet and I hate to sound like I'm picking one above the other or promoting one above the other.
You are in the right place to learn all you will need to know about tackling your first build, several very knowledgeable people on here that will jump right in to be of help in any way that they can to encourage you get things done in a way that will hopefully minimize failures and problems and result in a safe, fun and reliable as can be motorized bicycle.
Welcome to this great bunch of motor bike lovers and I wish you the best with your first build.