I would have bought DMC4:SE retail from the US but I'm not willing to import it. It's ridiculous THAT gets digital only and not DMC (or you know both physical). I'd even accept retailer exclusive, I'd happily have a bunch of games go Amazon only than digital only.
IF it helps Neptunia Rebirth 2 is a bit longer than 1 (reb 1 was about 20 hours, most of the other games go around 40 hours for first clear), so Nep and Raiden should be a good chunk of game. Both options are good.
If you get Neptunia 2 I recommend either not going for trophies, or using a guide for endings (saves in the right places and planning make it WAY less painful) and the $1 DLC to "cheat" in the Stella minigame. You will HATE Stella. If you don't care for trophies, basically ignore Stella and go for the true ending.
If you don't like grimdark stuff, do not let Planeptune's shares go over 50%.
And you can always ask for help or discuss the series in general in the
Neptunia Community OT *sparkles*