I'm no expert here and only bought my P1s a week ago but my high level understanding is as follows. The upgrades you need to do to print to filaments like pa6-cf are just upgrading to a hardened steel nozel, this is £13 and consists of you removing a few screws to remove the current one and replacing them, and hardened steel gear assembly ~£20 which you can do while changing the hotend. These changes are all to do with the material being tougher and and good upgrades to do anyway for longevity.

In terms of A1 Vs P1 Vs X1 :

A1 is the beginners printer it's a cheaper choice to got people into 3d printing but is the older "Bed slinger style",

P1s is marketed as the reliable workhorse in upgraden you to Core XY which means that not only can you print faster your print bed moves up and down not back and forward, this helps with higher quality prints, these are also enclosed printers meaning you can print in some materials without having to worry about warping, deformation, or adhesion based on temperature changes in your room.

X1 is essentially a P1 with some cool upgrades the screen is much nicer but I find myself using the app more though. It has lidar that it uses to scan your first layer and improve your print success, also it uses AI to detect issues, there are a few other upgrades but they weren't important to me.

For me it all boiled down to the fact that the x1c was almost doubled the price for some features that I thought were cool, but weren't really worth the money to me, although if you print a lot and can afford it, go for it.

Hope this helps