Yes. Only post-game sidequests are Dragons Den and the Soul Shrine. Both can be went thru regardless of completion.

Levels do very due to Esper stat enhancment, equipment and magic. But a good level base would be about 40.

Good equipment would be Light Bringer, Hero's Sheild. As described above, but also a green beret and Force Armor. Minerva Buster for the ladiesand Snow Scarf for Gau.

Gau is a great character is you know how to use him and what to chose depending on any conflict. Locke is great because of his mug ability with a genji Glove. Umaro after obtaining the Berserk Ring and Blizard Orb, no elemental magics will hurt him. Only non-elemental. Though characters are all up to you. You will like some over others so chose to your liking.

Good Relics would be Ribbon, Zephyr Cape, Prayer Beads (with the Light Bringer, Hero's Sheild, Green Beret and Force Armor, you are nearly invincible), Genji Gloves, Miracle Shoes would be well used.

Just prepare for tough Magicle and Physical beasts. So lotes of good magic, a good level is recomended. Gau is good for his spell usage during Rage that doesn't lower his Magic Points.