I'd need to hear more about you really want. What do you mean by kit? Does that mean you want a production gun with accessories or a production gun in parts that you assemble and finish the stock? Or a bunch of parts from which you would build a gun?
Since you mentioned shooting sabots but want something traditional looking, I would suggest a Lyman Great Plains Hunter. This is available in .50 caliber with a fast twist barrel. It should be available in left or right hand and percussion or flintlock.
Another site you might want to look at is Dixie Gun Works. They have a number of reproduction guns to choose from. If you want something more custom, look at Track of the Wolf's website.