Yeah, your paying them $399 to do the work for you. Post up links please, I want to laugh. But seriously, if you can post a link, I'm REALLY interested in seeing what you get for $400.
If you can read a simple electrical schematic, you can build it yourself. If you can wire a 3 way switch in a residential wiring setting, it should be no problem.
I built a similar phase converter with a push button start and stop years ago, 3hp converter running 3 mills and a lathe, no problem. I WOULD NOT START THEM ALL AT THE SAME TIME! But yes, all 4 have been running at the same time without issue. The lathe is 3hp, one mill is 2hp, the other two mills are 1.5hp bridgeports.