Right now, my advice is as follows:
If you can afford it, get Brissa units. They're one of the most accessable unit sets that will be useful for some time. If you have the capacity to get Circuray units, Zeinesis, Whiteal, or the clifard units, use those.
As far as your weapon is concerned, 13* weapons are nice to have. Really, any of them are good weapons, but you'll do a party more of a favor by learning how to fight with your weapons, than equipping "best in slot." Learning when to dodge, or block, and when to DPS-- learning the bosses attack patterns are far more important than any of that. A ground up Sigma series weapon to +35 and a keen understanding of the boss and level are far more valuable than a few extra points of damage.
That said, that recommendation does come with a few caveats:
Firstly on the list of 13* easily found weapons are the Unique weapon's badge weapons. There's nothing really to write home about on these weapons beyond the fact that they're 13 star weapons. Note: If you're a bouncer main there isn't an option here for Jet boots-- but, you get these for taking down rappies on a high interval.
Right now in game There is a limited quest called "SP Breakthrough Training: EP 1-4" going on that has a high chance to drop a 13* weapon series known as the Fornis Series. Because of this, they are insanely inexpensive, selling (On ship 1) in the 100-300k price range.
Next in line are the Homura series and the Ray Series weapons. By talking to Badge & Memory Exchanger Nanon, you can trade in Rising Weapon Badge 2's for Ray weapons, and 3s' for the Homura series weapons.
What's more, there is a collection folder from Equipment officer Prin that can allow you to get a guaranteed Revolsio weapon for free, and of the type you need/want after you do a handful of quests, and loot some stuff.
All of these weapons, (specifically) The Ray, Homura, Revolsio and Fornis series weapons are great to good 13* weapons, that don't need to be affixed right now, and are on the inexpensive side of cost.