Apparently even though the Quark Monster is toted as 6s compatible, it can burn up running 6s. Make sure to do the heatsink and cap mods before you try to run 6s.
Second, as far as you requirements 'good runtime, good torque, good highspeed' - well thats pretty much what we're all after isnt it? :P have a look at
http://scriptasylum.com/rc_speed/_top_speed.html to get some top speed estimates with different pinions - I think aiming for about 40mph is usually a good place to start, and adjust from there.
What capacity are your batteries?