I've ran steel and plastic, I prefer the steel, because with the right mesh they last much much longer... the one thing with running hardened cbs and spurs is that I always set my slipper a bit more loose than normal. doing this I still have the original spur that came with my xl, and I put at least 3-4 gal on it so far
as far as what backups.. uphmm one of the first things i would do is get the integy rear hubs, to eliminate your rear tie rods, then when the arms break get rpm arms. Another upgrade to consider are new servos, and the ofna throttle mod. I know this isnt a list of backup parts, but if you take care of these mods early on it will save some head aches..
now for spares i always keep a spare set of tie rods, pullstart, front/rear skids, hingepins, body clips, wheel nuts, set screws, glow plugs, shock towers, body mounts, I know I'm missing something.. tomorrow I will go through my box and ill list what i keep on hand when i run my savage. Of course you may not run yours as hard as I do mine, but I have broken at least 1 of everything listed above with my xl.