are we allowed to buy extra kits like the extra metal or sprocket and chains set? couldn’t find this in the rules.
Yes, there is no limit to the vex hardware you can use. The only limits are on electrical components which are limited by the controller’s capabilities.
You can use any amount of metal hardware and/or chain & sprocket kits. 
Good Luck!
It depends on the competition too. My team did a comp and there was no limit on anything except only one controller per robot.
another comp we had a 1 kit limit with one add on
it varys per teh rules
for the OFFICIAL FVC (not the ones mentioned in the last post), yes, you can use all the hardware you want.