just talk to everyone in town before you move on, that's the best thing you can do.
most of the real sidequests are on disc 4, you know? the typical "there is the final boss and there these places you can go, do whatever you want".
there are no classes here, you pick your character based on their skills and if you don't want this character, you can let an immortal like kaim steal his skills.
for example, mack have a skill called combo (2x attack at one turn) i love this skill but i don't want mack so what do i do? let kaim and seth learn it, simple as that.
as for items, seed slots are the most important (gives you more skill slots for immortals) so you may wanna check a guide for a list, but as i said if you take your time and talk to everyone you won't miss any or maybe some, there is also 99 normal seeds you can get and exchange them for items and accessories, check gamefaqs if you don't wanna miss any.
also, always check the items bosses carry, you might find great stuff to steal like a seed slot.
final thing you must know, all items you missed can be won in the auction house later at disc 4.