Hey P.chraca!
I'll do what I can to answer your questions, but I can't assure you that it'll be the best or right ones you're looking for. Regardless, I personally very much enjoyed my play-throughs of this game on the PS4, and am super excited to see others, such as yourself, getting an interest in the series as well. There's no need to have played previous games, as each one starts off as a new story and adventure with new characters, weapons, location(ish)*, and etc. There may be some cameo's and references to previous entries, but pay no mind to those as it doesn't affect the gameplay whatsoever.
Now, to answer your questions:
1. As the game states, you and the rest of the passengers are castaways on the "deserted" island of Seiren in which you're all attempting to escape. There is actually quite a bit to discover, as part of the game accomplishments is 100% the map. That being said, you are on an island, and there's really not a whole variety of different locale to visit - despite that, what you are given I found is very well detailed and very engaging. They elaborated their designs in their own way, and even if there's not a massive array of different locations, there are still plenty of interesting and unique niche's to explore.
2. Overall, I'd say about 40-60 hours (depending on your choice of difficulty). I know that I took a bit longer as I'm the type to spend hours trying to 100% games in the first go.
3. There is some replay value, moreso for me as this was my first Ys game and I found myself very interesting in it's setting and characters (esp. Hummel). But, there's nothing really to add to another playthrough, minus maybe playing on a harder difficulty or completing side quests that you didn't manage to get before (or getting better scores on them, if you'd like). Here's a basic list of what you can carry over in the event of a new game:
- Character levels
- Skills and skill levels
- Equipment (excluding weapons and armor)
- Adventurer's journal
- Materials and ingredients
- Map
- Treasure chests and gather points shown on the map
- Three empty bottles
4. To be perfectly honest, there's not a whole lot of challenge on normal. Although, I have been playing games (specifically RPG's) for most of my life, but the controls for this game aren't really too difficult to master. You level up fairly fast, and the only real problem you may have is running out of items and HP on the long dungeons / battles, so watch out for that.
(Don't forget that you can always change your difficulty mid-game if you find you current one too easy or hard!)
Hopefully you enjoy the game, I'm definitely glad that I picked it up when I did. Good luck on your adventures through the island of Seiren, and I hope I was able to answer at least some of your questions!