Yes, healers use RES instead of INT for healing.
yes guns use HIT and SPD combined.
main campaign is a good 30 hours, you will not be over level 50 or 60 by the time you finish it unless you grind or spend time in the item world. but the main campaign is not where the game ends.
I'm a big fan of the series, you should play 1-3 if you like 4. They are all pretty much the same.
There are also other Nippon Ichi games you'd probably like: Phantom Brave, Makai Wars, Soul Nomad.
Non NI games you might also like: Fire Emblem, Stella Deus, Record of Agarest War, and so on.