This is a tough one...
While you can build your own frame there is a lot more to consider than welding expertise.
1. When you are done, if you intend to sell them you will need to provide an MSO with each one, otherwise people will have a very difficult time registering them.
2. You will need...
The manufacturer you are thinking of is Bourget's Bike Works
http://www.bourgets.com/pages/home.html
That will cost some serious $$$$, if you can even buy just a frame from them