Just a couple of comments (I think we have touched already most everything essential, otherwise)
1) Sharing a VFD between machines: I second the general feeling that it is better to have dedicated VFDs and, if sharing, not only the motors should be equivalent, but the "mechanics" of the two or more machines should be very similar. As pointed out, it would be extremely handy to have instant braking on a mill for power-tapping. If you plug a surface grinder (or even a lathe with threaded spindle) on the same VFD, life could become very exciting, with grinding wheels and chucks unscrewing themselves when you stop or, even worse, you do an accidental instant reverse.
2) Instant reversing is fairly demanding on the VFD. An external power resistor is a must. Given that you need that kind of braking only with emergency stop and power-tapping, and given that most VFD allow you to set different braking values for regular stop, emergency stop and jog function, I'd suggest to use the jog function for power tapping: you put in forward, push the jog button and release it when the tap is at the depth you want, flip the selector to reverse, and push your jog button again (or start the motor with the regular start button, since you don't care if the spindle keeps spinning when the tap is out).
3) I find VFDs extremely useful on lathes and grinders, a bit less useful on milling machines. Especially if you have/plan to have more 3 phase machines, you might consider investing in a decent rotary phase converter (RPC) instead. Or, if your second 3 PH machine is/will be a surface grinder with a motor larger than your BP, you might spec out the VFD for the spindle of the grinder. For now you can use it on the mill and, when it comes the time to power up your grinder, you can evaluate how many times you have adjusted the frequency on the mill and decide if to buy a second VFD (for the mill) or a RPC.
4) As mentioned, VFD do not like any switch downstream and, except for special conditions (e.g. conveyor belts of similar load run by identical motors), they cannot run multiple motors at once.
For instance, in the case of a surface grinder, it's common to have a second motor for the feeds and a third one for the coolant pump. You would therefore need a VFD for each motor. The spindle motor is the one that would profit best to be driven by a VFD (e.g. slow ramp-up curve, limited braking, speed control, etc.) whereas the other two could run at constant speed (perhaps, depending on the design of your grinder, also the feeds motor could profit of a speed control). Therefore, having a combination of RPC and VFDs would be the best (and RPCs, contrary to VFDs, work better the more motors they run at a given time-within their load capacity, of course!).
Paolo