Two part question, plz advise

[Stepjam]

Eureka: Starts in Kugane, the quest is "And we shall call it Eureka". Unlike the other two, you don't need to be overleveled to do the Eureka weapon because it all takes place in an instance that caps your level at 70 (you do have to be at least 70 though). Eureka is a different beast than the rest, inside the zone eureka, you have a brand new level to grind (going all the way from 1 to 60 over the course of 4 zones). Each zone has a grind to upgrade your weapon (outside of light farming in zone 2 and getting logos actions in zone 3, it's largely just gathering crystals from killing special enemies). The first weapon will take a fair amount of time to get, but subsequent weapons will go faster because A:You'll be at cap level already so don't gotta do that again and B:you'll have gotten the logos actions already so you don't gotta do that again either, just gotta farm the materials needed. I highly recommend you look up the finished products for the Eureka weapons before committing, because many players consider them very hit or miss in terms of how they look (and miss more often than hit, though there are a few cool ones).

As a final note, you can't get a weapon for a class that wasn't out when the weapon series was current. So for instance, you can't get a zodiac weapon or anima weapon for samurai or red mage since they came out during stormblood.

Edit: Also as far as the current weapon series goes, note that the Relic Weapon only becomes best in slot on the very final update, until then it's considered catch up gear (the first step was ilvl 485 at a time when 490 was the best you could get without running savage, the new step will likely be 495 since players can get 500 now for their tomestone weapon) The current best weapon is the weapon that drops from the final fight of the current savage tier, with the upgraded tomestone weapon being second best.