The "Team" boots from Nordica are Junior models. They are softer than the Nordica adult boots with a Flex of 100. FYI, The Doberman Pro and worldcup boots range from 100 to 150 on the Nordica scale. The 120 can be used by very good, or very large, recreation skiers. The 150 Flex is for racing and not really good for anything else.
"Good flex" can mean a wide range of things, but the flex of the boot should be consistent through the range of motion. The boot should smoothly resist flexing at the forward end of the flex range.
Stiffer, of course, is not always better. The boot should flex and allow the skier to absorb the changing or bumpy terrain. Also, the ankle must contribute to balance and not be frozen by the boot. A boot without flex would make balance and skiing impossible.
I would look in the 70 to 100 range. My daughter uses the Nordica Beast set at 110, her boot is adjustable up to 120.