To answer you questions in order-
Yes. A 6i25i (either PCI or PCI-E) (If you are computer savvy, network savvy and linux savvy- you can use a 7i92H or M)
You don’t need an OS, the PathPilot ‘installer’ is a disk image. So whatever system you run the USB key against, it’s going to wipe the machine. As for the machine, a basic Intel machine with either a ethernet port or USB WiFi and a HDD that’s 40GB or bigger.
See answer to question one.
In addition to the 6i25, you need a parallel (DB25) breakout board. A basic one that’s is NOT meant for Mach3. This way you can wire your existing drivers to this. Fortunately the pin out for the main axis (XYZ) are in the common locations. The A Axis is wired differently for the 440 in the config (Mainly due the fact that the 440 does not have a rotary option) But this can be changed.
The next step is reconfiguring the stock 440 config to work with your machine.
This is going to require some linux knowledge. So if you are going to do this- be ready for that.
PathPilot is basically LinuxCNC. So the underlying files are similar. The main things to change is the step pulse (to match whatever method you are driving your CNC.) for all your axis, setting the max feed rates, Axis direction, size of the workspace etc. Also configuring endstops. Thou you can use PathPilot with out endstop- its a pain in the ■■■. Just add them. Lastly is spindle control. You can do speed control with a VFD, SuperPID and basic on/off. Anything else that requires a unique PWM signal is going to be hard. This is due to Tormach removing alot of underlying libraries from LinuxCNC to make the package smaller.
I’ve done it. So it’s possible. But it’s not going to be some tool or wizard to make the changes. This is going into 2 configs files and making the changes. And knowing what to do if the changes you make crashes PP on boot.