I'll ditto what lakeside said. I only bought the VFD because I needed 3-phase. I had no idea about the other benefits. BUT now I am so use to using it to slow down my speed ups, speed up my slow downs, and fine tune my speeds, I don't think I'll ever go back. You can make a rough cut then speed it up and make a final pass without ever having to deal with the pulleys. The only reason I can't get away with it on my lathe is because they are pretty expensive for 5+ HP... and because I picked some inexpensive parts to build an RPC . If you get one for the mill you won't be sorry, even if you end up needing a converter later.
in order to learn... not save money.