The M18 is a 150mm wheelbase Micro.
Body choice is personal, but from experience I can say the HPI Micro shells are superior in quality, finish, masks and decals compared to the ProLine/Protoform shells that XRays ship with.
As a Mod on YourMicro.com I have a vast collection of Micros, including an XRay M18. I use the crappy Ferrari shell the XRay came with for bashing...
My favourite shells have to be the Corvette C5R and Mercedes CLK
One of the YourMicro.com Members has produced some awesome new boyshells in 150mm - ideal for the XRay.
More info on them can be found here