Car starters have been used on single place gyros and they are about 6 ft-lbs with an initial peak of 17 ft-lbs. If a drill motor isn't enough they make medium duty battery powered chain saws that rival gas powered. A motor controller will be key to avoid full torque being applied to the mast at start.
Of course the quicker you want it to reach full rpm the more torque required.
That's from experience - in my early days we used typical Dominator hydraulic prerotators where power for pump and motor (Gresen/Parker MGG20025 pump and 20020 motor) is known from their tech data if one knows rpm and pressure. Thus power consumed on given rrpm could be easily calculated.
Please do not forget that power required for prerotation grows in cube law. Say, to prerotate DW to 100 rrpm one needs not 2 times less power than for 200 rrpm but 2^3--->> 8 times less.
Though 100 rrpm are useless for DW28 rotors - they need at least 150 rrpm to begin safe take-off roll on a parquet surface and even more on rough one.
Two colleges are doing two pre-rotators projects for PRA. In horsepower terms to spin 30-foot blades, it takes 3HP for 150 RRPMs. It takes two-speed transmission and 3HP to spin it up to 170 so far. The class ended before we change the gearing for the 2nd time to learn just how fast we could spin them. With the transmission, it is like 5HP.
The 2nd college just started the phase of ordering the parts for a lightweight state-of-the-art prerotator and the math show us that we need a 7.2 HP motor to spin the same 30-foot blades to 220 RRPM. I have most of the parts and an 8HP motor sitting in my living room waiting for the SDSU semester start 8/27.
On my Bensen with a 23' Metal Bensen blades, My Kobalt drill motor would reach a total of 110 Rotor rpm with the rotor tilted full forward and no assistance from the relative wind through the disc.
More than enough for a short takeoff launch.
It really messed with their minds when I taxi out with the drill motor hanging off the front of my seat. When I spin up, I just toss it overboard and go.
Different rotors make very big difference in prerotation power required. DW rotors with their positive twist need much more than any non-twisted rotor of the same size.
I once replaced standard Xenon 8.40 rotor and installed DW on this Xenon to see what would be the difference in flight performance. First thing which I saw was that I could only get 180 RRPM with DW28' while same prerotator easily turned Xenon rotor up to 240 RRPM.
Some years later I made same thing on a Calidus - with exactly same difference for 8.40 Auto-Gyro rotor and DW28'.