Its my first MR2 and I'm keeping it stock, I went to buy new rotors and pads from Repco the other day and they said there are two rotor thicknesses available for the 91 model based on the engine power output? Where will I find power output on the car, should I get the thicker or thinner rotors if it doesn't matter? I don't have the knowledge yet to modify anything so I'm just replacing whats there. Any suggestions for a newbie?

They have different thickness & diameter. You will only be able to use the one that matches yours, they're not interchangeable.
 
Earlier model fronts are 258mm x 25mm
Later model fronts are 275mm x 30mm
 
You should measure your rotors to make sure u get the right ones.

Don't worry about power output.
Larger brakes/rotors were used after 11/91
One quick check may be to look at the wheel/tyre plate on the driver's door post.
14 inch wheels = smaller rotors.
15 inch wheels = larger rotors.
 
However probably best to measure anyway.
 

Thanks guys easy sorted.

Yeah, Repco look at a **** book that tells them that we have the US spec stuff. Ignore it. Find the stuff you need online and go in with pats numbers.
 
They're still stuffing me around with a set of rear pas for mine, so I just went to a real brake shop for the parts.

Good advice. Cheaper and more choice online