Lathe controller

Yes ,~10 MHz is the limit on the 7I76E

The latency is not very important as long as you can access the card every millisecond

Step rate settability varies proportionally with the step rate but is about 1 PPM at 50 Hz
( rate generator is 32 bit accumulator running at 100 MHz )

Sry andy your answer confuses me.
if the latency is 0.5mS I still have 10Mhz ?

Yes

So accessing once every mSecond shouldn't be a problem with any decent PC guess.

Most PCs will work (Though very slow PCs like Atom D525s are marginal with Preempt-RT)
For fanless systems, Intel J1800, J1900, N3150 based PCs are good choices as are the AMD A4 series

I have a Mesa 7i76e card and am driving it with a Gigabyte Brix J1900 Ultra Small Form Factor PC that fits in the palm of your hand. Latency is quite poor but still OK according to Mesa. Here's what I am achieving...

I am using Longs Motor DM542A Stepper controllers set to 25x Microsteppping. So on my 200 ppr steppers, thats 5000 pulses per rev. I can run these reliably at 650 rpm. So lets do the maths:
200 ppr x 25 microsteps x 650 rpm = 3,250,000 pulses per minute (or 3.25 mHz). Your 120kHz will be a walk in the park.

What I achieved is still well under the published maximum step frequency. I think the limiting factor in my setup was the step lengths (which meant there is a limit to the number of pulses you can fit in a second). After settling on these specs, I reduced the step length from 2500 usec to 1500 usec but never bothered to see if I could push it further afterwards. (going below 2500 usec is not recommended) Anyway, if nothing else, I proved that there are other physical limits that come into play well before the 7i76e reaches its maximum step frequency.

Before you commit to any certain interface hardware, you should consider, and determine the hardware you will be using with the machine, because some motor drives are better controlled with certain hardware. Is this lathe a conversion of a dead CNC or is it automation of a manual machine. If it is the resurrection of a machine that already has motors, it is often best to reuse the old servos. Which, unlike Mach, Linuxcnc can usually easily control with the right hardware.

It is neither. It is a new build one. I just purchased the spindle and motor bed mechanics and electrics will be made by me. So i am free to use whatever i like and can afford.