I personally would buy a kit for a reputable company/person if you are hard set on building it yourself. If you ever run it to problems they could help you out. Some of these other companies that sell "kits" don't offer real support after the sale.
If you have no experience building an Allison and have the proper tools and information, I will say get it built by a professional. I have read people buying kits and running into trouble with no support. They try to save money but end up spending more money because they have to take it to someone to fix it. You can do what you want but that is my opinion.