The best thinng to do is to just find a picture on the internet of the watch you are interested in and send it to one of the dealers. their websites are really just for reference. They are not updated regularly for the most part and they do not list all of the watches that are available. I don't even bother looking at the websites. I just send an email with a pic of the watch and ask if it's available and how much it is what movement I want in it, and how they want me to pay. That's all you need to do.
As far as why he would have put a genuine movement in it is because a lot of us like to modify our watches, by adding gen parts to them, or any way to improve them. It's not something you have to do. but some people like to mod their watches.