The 20C and 35C ratings are discharge ratings. The 20C pack might be a little weak for an 1/8 scale, make sure the pack doesn’t get warm while running. an indication the lipo is being overstressed. At the least results in poor pack life, but the real concern is a thermal runaway and nasty fire and toxic/corrosive smoke, not good.

As mentioned look for charge ratings for the packs too. 1C / 5 amps charging if you can’t find higher ratings is the safe way to start (doubt if the 20C is any higher than 1C charge rated). Use the balancer it’s faster and safer.

I have the earlier version Duo3, it's great, charges fast and has advanced features that I really appreciate. Solid choice in my opinion, especially for electric 1/8 racing as erevo mentioned.

A 1900 motor is a good choice, plenty of power, even 1700 is enough for most all. Good move not going with more motor in my opinion. Good luck!