Ok brands. Ace Gen, CoreRC, Peak usally solid but always check your batteries arrive balanced. *
*You need a battery meter to watch the lipo cells and monitor them.
1. No limit, drive time is down to mha, the size of the battery tray is your limiting factor for mha thus also volting up will also give more run time.
2. Your FTX should have a jumper for lipo.
Lipos will boost your speed and also can melt brushed motors with ease due to the current delivery being much more than nimh, ease into it.
Lipos will boost your runtime by 30% over nhim (weather dependent) had 9000mha 2s go for 4 hours. Ideally dont use your lipos in - temps it will kill them.
3. A dual charger, becuase nothing is worse than a dead battery, always take two with you.
dont skimp out on reading the threads, follow saftey advice, people have lost their houses to lipo fires, dont be fearful of them, just be smart.
If unsure, ask ask there is never a silly question.