5kg for upper body sounds about right. 10kg for lower body may be challenging- it's what I try to use, but I usually have to switch to 8kg
about halfway through the workout. But I'm quite old - if you're young and fit, 10kg may be totally doable. Do you have access to a gym where you can try them out? If you get 8kg, you'll eventually use them for upper body and progress to 10-12kg for lower.
I think one has to accept that if one takes this seriously, one has to upgrade periodically. Or go with adjustable dumbbells, but these can be a bit fiddly. I currently have 4 sets, ranging from 5 to 12kg and I use them all, depending on the exercises and how strong I happen to feel on a particular day. Dumbbells have become quite expensive - but still way cheaper than a gym membership.
As for Caroline's brand of HIIT, I get you 🙀. I tend to save those for days when I'm in bad ass mood. The rest of the time I substitute something by the lovely Anna... I often add on a short Growingannanas tabata workout to my daily dose of Caroline because I feel I need more cardio - and I hate running...