If your child doesn't show signs of travel sickness now (if not actual puking then distress in the car) then she is unlikely to suddenly develop it - not all children are travel sick. I was horribly travel sick as a child but only one of my three gets it.
But you don't need to justify your decision to FF if you want to do it. As long as you are aware of the difference in safety, make an informed choice and be confident in it. The younger the child is, the more of a risk it is - 15 months is an absolute minimum, it does not become significantly safer at that age.
Personally, I don't think the Joie 360 is the best for FF - I would prefer a seat by Britax, Recaro or Maxi Cosi or possibly Cybex. The Britax, Cybex and Maxi-Cosi spin seats tend to cycle special offers, so one of them is usually around the £200 mark, which is a bit more than the Joie seat, but Joie 360 Spin is the cheapest spin seat that I would trust (other than Graco Turn2Me which is the same seat anyway) and it is worth paying a little bit more to get an easier to use harness/inserts, since the straps are really important to get right when FF.
Also, if you are highly likely to FF ASAP, e.g. around 18 months, then it may actually be better to just max out the infant carrier and keep her in it until it's totally outgrown, then swap to a purely FF seat like Maxi Cosi Titan i-size - these are also a little bit cheaper sometimes going to £160ish on offer. Because they are only for FF, they are more optimised for it which is beneficial since younger children are more vulnerable in car crashes when FF. The extra distance from the seat in front, the top tether, the fact there is no moving base are all advantages.
OTOH if you plan to see how you go with RF, stay RF until around 2, or alternate for higher-risk journeys, there may be a benefit to a spin seat which includes a RF option. It's just about weighing up what you think you will be more likely to do really.