If it is the E36 M3 and you want the 6 speed, then you will need to find a 1996 - 1999 Euro Model + that will come with a 3.2lt inline 6.
The E46 M3 and R34 GTR will have 6 speeds stock.
Power range?
Light mod?
Can't say much about the cost of modding the M3's but I assume it is in the similar price range with the R34 GTR.
You'd want good quality parts / upgrades so prepare to send a bit, IMO.
I am yet to had a drive in a M3, I really do like the E36 and E46 M3's tho.
I remember the 1st time I saw the E36 M3 (1993) and thought how nice of a car it was and still till today, a nicely done up E36 M3 looks great but it is a rare sighting tho...
Half the time it is all riced up E36 318i's.
As for performance, the spec's are pretty much inline ball with the GTR.
But the E36 M3 will be the lightest of all then the R34 GTR then the E46 M3.
Boost controller / ECU tune, exhaust on the R34 GTR.
M3 you can do exhaust, air intake, ECU tune.
+ it depends on how much money you want to spend