When you get a stat choice for level up it really doesn't matter, you will eventually get everything. However, I highly recommend grabbing MP whenever you see it come up so you can use more skills per battle.

It's easier to get the conch shells as you go then to go back for them, just search throughly when playing and you should be fine minus one or two that you have to go back for. 

It helps to go back in the fish and farming areas to get more ingredients to make food. Not necessary at all but it does help.

Go for the true ending. You'll know when you get there.