1. The difficulty is hard to very hard IMO especially the army you face in chapter 9, and the last army you have to face in chapter 10, I had pretty much OP units and still had a hard time beating those armies.
2. I think you could understand the story well enough without understanding the text but there are characters and secrets you might miss.
3. The length I think is about 50 hours to 100+, if you don't have to restart I think it will be 50 to 100 (which probably won't happen).
4. As for tips, here are mine:
* Make sure you're working on the pairings you want before you get far into the game for the 1st and 2nd gens, because unplanned pairing can occur and can't be undone, I think the love system is one of the most infamous mechanics of FE4 so be careful.
* Have a guide to get the secrets in the game, the secrets are pretty useful like Finn's Brave Lance, Ayra's Brave Sword, and Lachesis's Knight Ring, and some are very hard, if not impossible, to find especially in a first playthrough, unless you want to waste 100-300+ hours moving all your chracters to unsually spots on the games humongous maps, I would highly recommend a guide but it isn't necessary.
* 50 kills+ allows critical for a weapon, so you may want to use certain weapons more then others.
I hope this helps.