I would suggest "One Scythe Revolution" as a good resource for scythe information and equipment. I'm a little confused by "custom made scythes". A custom scythe blade of quality would run well over $100, if you could find someone that could make one. The snath, or handle, needs to be fit to the user, and the scythe blade mounted on it accordingly. So, "custom fitting" a scythe setup is a necessary part of making the system work, but a "custom scythe" is a different thing entirely.
As far as being a tool that will last a lifetime - depends on how you use it and care for it. Grass blades are kind of delicate, hit a tough weed or a seedling, much less a rock, and you can have a job getting it back in working shape. Use it as a pry bar and it's done. They rust overnight, even when you thought you were taking good care of them. Brush blades are stouter, they'll handle the weeds and some saplings, but still not pry bars

The snaths - handles break sometimes, and they take wear from the mounting of the blade. Aluminum may hold up better, not my thing. One thing about the wooden snath is that the farmer can pretty easily make it himself.
Also, Benjamin of Baryonyx Knives posts here on permies and is quite an expert on American style scythes. He refurbishes old blades and snaths and puts sets together for sale.