I dont have the armor stats hanfy, but the M18 Hellcat was not as lightly armored as an M8, but not as heavily armored as the M4A1, so pick your poison.
This subject has been debated time and time again... the M18 would bring a combonation of SIX things that currently do not exist in the game: high speed (45mph+), effective main gun (US 76mm w HVAP/HEAP second only to the 17lb'er and 88mm), low armor (more than M8 less than Pzr), and minimal anti-OBJ capability (it carried minimal HE rounds and no MG other than a single M2 .50 cal on top for a token AA defense), AND it would be vulnerable to air attacks (think of how fragile the T34x turret is vs enemy tanks, now apply that vs air cons).
Oh... and I'd be willing to guess it would have a rate of fire faster than the Pzr IV but less than the M8.