It's true that inherently IPS is slower than TN but the majority of input lag comes from input scalars within the monitor and internal buffering.
For example something like a 27" 2560x1440 Korean IPS with no input aside from a single DVI port will have nearly zero input lag, in the range of a few ms. Other IPS like Dell Ultrasharp U2410 also will have about 10ms or an HP ZR2740W which also has about 10ms, which is decent for an IPS or TN.
In fact the lowest input lag of any monitor AFAIK is the Dell Ultrasharp U2312HM which is an IPS with an input lag of reportedly down to .6ms!
Eizo Foris FS2332 is also an IPS with ~1ms input lag.
There are Korean 27" 2560x1440 IPS with overdrive and 120hz + capailities but they're often sold out and tough to find, but they'd be my ideal monitor.
BTW I am not talking about the commonly specified GTG response time, that's how long it would take for the pixels to flip from grey back to grey. I'm talking about the total time it takes from receiving an input signal to changing the pixels.
I have no idea how much graphics power he'd need but I'm sure it's hefty.