In terms of boot stiffness, there are too many factors for one-size-fits-all advice. I underboot my kid because even though she skates everyday, has all her singles, and has good power, edges and flow, somehow she is really light on her boots. She actually weighs a decent amount too, she is muscular and dense. If they are remotely too stiff, she can barely skate so she is in something quite a bit softer than recommended for her level/weight. She has never had to replace boots because she broke them down - it has always been from growth. On the flip side, my friend has a teeny, tiny lightweight little girl who chews through boots, so they have to go with really, overly stiff boots for her.
Too many factors go into boot decisions that what works for one person, doesn't for the next. I think you have to get through at least a couple of pairs of boots to really know what is and isn't going to work for you, and unfortunately, that process is expensive.