In my opinion, a separate HD is better than dual boot. Hard disks are cheap and is safer, since if one partition crash, it may kill the other also, which can't happen if you have physically removed the hard disk you are not booting from.

You can't disable W10 updates, you can only postpone if (if you have a pro version). The only way to disable it is if you prevent Internet connection, or through some sort of third party "fix". Also, why would you prevent W10 updates? It is better to keep the computer in shape, just like you do with a car or any other machinery, than let it slowly degrade because of no bug fixes and other things which may cause issues. The only thing I don't let Windows to decide is when to install the updates and when restart is allowed (if necessary).

If you are still running XP and not even tested W7 then you are WAAAAAAY behind and outdated. Of course, if you run Mach3 on PP then you are forced to use these outdated systems, since Windows stopped supporting PP with W7 64-bits. W10 is the best Windows ever made, it NEVER crash unless you do something or use some buggy software, or if you try to "improve" it's performance through different third party "fixes".

BTW, don't ever buy a modern car if you are afraid of software / firmware updates... Tesla doesn't even ask you anything, just updates. BMW asks, but eventually updates automatically and I don't know for how long you are allowed to postpone, but the maximum is to the next service, where they ALWAYS start with updates. Other cars get updated automatically, like Tesla, or like BMW, or if the updates are never pushed out through the network then it is done when the car is in for a service. So in the end, all modern enough cars get new firmware now and then for the last 5-10 years. The only thing I hate when my BMW gets an update is that I have to go through all the settings I made to make sure that the car is configured as I want it to be, not as the BMW engineer thought it should be. Some times they change things, but that's all, never experienced anything negative which came from an update.