Surely they aren't actually 75HP. That would be unreal.
Those motor are really 75 HP, 175 amps at 230V. A little big for most home shops
In a rotary phase convertor the idler motor needs to be about 3 times larger than the motors you want to run. So if you want to run a 3 HP motor you need a 10 HP idler motor.
View it! spend ten minutes going over it, then walk away and look at other stuff. Then go back and give it another ten minutes, and if you get chance, do it again. If the machine is basically good, dont worry about the rust, it will clean off easy. I buy machines like this(or used to, retired now) all the time, for pennies, spend a few days on them, and sell for thousands, and have equipped my worksop with a range of pristine machine tools for very little money. If there is a serious problem on it, you won't spot it first time you look! in many years of auction buying I have never regretted a purchase, only the ones I didnt buy! Transport is the bugbear, I was limited to 2 (english) tons!
I'd go 300 on that mill, if I needed one. It would need complete dissasembly, de rusting and repairs made to all the stuff you can't tell is messed up till you get it apart. As long as the ways are not pitted, the surface rust will come off with some phosphoric acid and scrubbing. If it belonged to a public works, you can bet it didn't get much use, so wear probably won't be a problem.