Hi @PeachPumpkin. Good to see another strength trainer join. It really is worth the time for your body and health!
I've been doing CGX for over 2yrs and I hope to carry on forever, health allowing! Definitely recommend a pair of 5kg, still use mine a lot. Prefer the smaller hex dumbbells to the larger plastic ones, they're much easier to handle. I think I quickly progressed to 8kg for lower body. I also have 10 and 12.5kg, but don't think you'll need them to start with.
Caroline uses really heavy weights for Iron so don't feel disheartened if you aren't anywhere near what she is lifting..I still wouldn't be able to match her now even after all this time training.