I started with Level 2, which are not particularly difficult. I think someone with N5 skill should probably start there. If it is too hard, then maybe try Level 1. I would skip Level 0 - they look too easy, especially for what they cost.
I’m working on Level 4. I do enjoy both Level 3 and 4. 4s are challenging for me N4 skill level. The packs call these intermediate level - N3 to N2 level, but this seems to be overstating the level needed a bit. Either that or I am selling my own skill short a bit. Considering the included CDs for listening practice, I think they are worth the asking price if ordered from Amazon.co.jp. Don’t order the whole set from White Rabbit - that price is just insane. If you order them a few at a time, it won’t feel like too much, then maybe sell them off later.
I would also recommend the Magic Tree House series of light novels. The grammar used is pretty simple overall - but they will be challenging - I will call them “Level 5 Graded Readers”. The amount of vocab used is very large compared to the readers, so you will need to stop and look up stuff constantly. I like them better than Yotsubato because of far less colloquial language used and far more material for the money.