If you're going brushless with your RC18 and don't want to be replacing diff and input gears then you will want to purchase some of these.
http://www3.towerhobbies.com/cgi-bin/wt ... LXJZH4&P=Z Use them to shim your diff gear closer to your input gear and your gears will have a much longer lifespan. I have been running BL/Lipo in my 18B for quite some time, and have yet to strip a single gear with 2 shims on each diff. Keep in mind each vehicle may be slightly different. I have seen diffs that only require one shim on each diff, and surprisingly enough I have seen diffs that required no shimming. It's all about the tolerances of your particular gear case. If you need any assistance or have any questions just ask, and I'll do my best to walk you through it.