The 80s sounds about right. I think my party was around that level when beat the game. I'll give a bit of advice though: there are 2 optional Monado Arts; Eater is kinda situational, but Armour is FANTASTIC. Be sure you get Monado Armour when you reach the Fallen Arm; it's incredibly useful and only takes 2 rather easy sidequests to get. Eater on the other hand takes HOURS to get.