bill pretty much has it covered.
1) varies. i get maybe 10 mins on a track from a 5000mAh pack and i can bash for a little over an hour on the same pack.
2) IMHO, upgrade what breaks. rpm is nice for things like arms. alum for caster block. mips or tekno for cvds.
3) nothing really. maybe change of gearing and change of how you drive it. LCG chassis is always nice but not needed.
bills link should be good help!
Tamiya DF03ra Lancia Delta | E-Revo | F350 SCX10
Current project: Carbon Fiber Vantage based E-Slayer