I think the stock spur is an 86 and you get a 19 and 23+ something with the truck,so your in the right ballpark.I use a 10.5 in my Slash,it``s just right for 1/10 tracks[19 pinion],but needs a bit more for 1/5 scale tracks even with higher gearing,no overheat problems and 20 minute runs on 4800 lipo
A 9.5Turn BL motor should be perfect.
I have raced,thrashed and bashed my truck for over 6 months and only broken one plastic rear hub [about £1.60 iirc] no cracks in the shell and still looking good.
They will take some very serious abuse,here is one on a 4 cell lipo setup
http://www.youtube.com/watch?v=9UqBpCK8Nf4