NEW Router / Plasma / Laser / Vision System Design - Which Mesa Combination?

You are correct, the 5i25/7i76 combo or 7i76E are easier to set up, normaly you have to wire it and power it, nothing more. They are perfect for most machines. You have only one cable with the combo, but thicker and shorter, ethernet can go quite far with a cat6 cable.
If you plan on using stepper motors and drives, that is the best solution.

I want flexibility incase I run into problems. I have on my desk bldc with quad encoders with ano log drivers 0-5v. Also available is DC servo disk with also similar quad encoders with pwm driver. I am building a high precision high count system which of the two gives more headroom for speed and precision.

For PWM/DIR control the 7i52 probably is the daughter card you want. It can alternatively be configured for step/dir control.
The 7i76 is, as far as I know, pretty much only used for step/dir and does not have encoder inputs.

PCW is suggesting the 7i80 because the ethernet-connected cards have better noise-immunity, which is good for a plasma.

Configuring the 7i80 is a little more difficult than configuring a PCI card, but the flip-side is that the Linux (rather than LinuxCNC) setup is somewhat easier. You basically install a normal version of Linux and then change to the preempt-rt kernel. This might have large advantages for PC hardware support, the Live-CD (and all RTAI version) kernels are rather old now.

Follow the instructions here to get a version of LinuxCNC that can work with all Mesa cards (the LiveCD RTAI kernel can not work with ethernet cards). linuxcnc.org/docs/2.7/html/getting-start...th_preempt_rt_kernel