You may want to look into that.
Allowing that much room will certainly leave atleast one car far more capable than the rest.
Plus, I'm looking to make it an arcade run for the sake of saving money as well as tuning ease.
Getting similar pwr is a good start, but getting nearly the same pwr is a better start.
Also, I want to leave settings out of a race like that.
Tuning for the lap might take far more time than the lap itself. Plus, I would really like to give everyone an even chance with cars at that level... Tuning a semi-racing suspension on a street car is one thing, tuning a fully customizable racing suspsension on a 1000hp racecar is totally different.
I like the idea behind your comment though...
Leaving a great deal of room for car selection may give us the chance to really play around with the cars and in the end, hopefully give us an idea of just which cars have what strengths.
So to wrap up...
Arcade mode preffered.
Moderate power levels.
Nearly dead even pwr.
That's a tall order, I know.

However, I am sure that with a bit of testing and a bit of patience we can find a great combo. 👍
Btw, doing
1040kg min and 550hp max in arcade would make these cars availible (but not limited to these cars)...
Ford GT, Oreca Viper, C5-R, NSX-R LM, 350Z LM, RX7 LM, McLaren F1 GTR, Zonda LM, Nissan GT-R LM, RX8 LM... And many others of course.

(probably a bunch of the JGTC rides 👍 )