I recommend level 60 if you are going to take out (SPOILER)Vayne

For the weapon, (SPOILER)Excalibur, Tournesol, Zodiac Spear, Scorpion Tail etc. all make good equip, but they are not for buying, they are mostly found in specific place ((SPOILER)Giruvegan - Great Crystal) (Unless if you count Bazaar Goods).

It doesn't matter which equipment you choose, just make sure they are top-notch. For example, (SPOILER)I equip Golden Axe, Aegis Shield, Dragon Mail, White Mask and Bubble Chain on everyone.

LASTLY, don't go to (SPOILER)Bahamut until after you get all Espers, finished all mob hunt and finished all of the sidequest. Some of the Rank VII-VIII Mob Hunt are even harder than the final boss.