No matter what mill you decide on, buy one, don't attempt a build. It's not like putting together a router. It will cost you far more and do far less if you even get it to work at all. There is a very good reason there aren't any do-it-yourself mill plans.
I don't say this out of any lack of creativity or adventure. It is just a necessity. I do build special purpose mills from scratch and 1) they just cost more than a manufactured mill in any case - economy of scale is a hard thing to fight, and 2) it requires, at minimum, already having a working mill, rotary stage and lathe in your shop to effectively to start a new build of any of those items in the first place!
If you want to save money, buy a mill. If you explicitly want the experience of building a mill, you have to buy a mill to do it with. Either way, same answer. Buy one!