Using feats, abilities, and other tricks and mechanics, what is the lowest level we can get a character to cast a widened fireball?
I have a 6th-level character with the Spell Specialization (fireball), Varisian Tattoo (evocation) feats, the Gifted Adept (fireball) trait, the half-orc bloodline, and a goblin fire drum. This allows here to deal 10d6+20 fire damage with her fireballs. I'm looking to...expand on that...and do it at as low a level as is possible.
At fifth level, with a metamagic rod?
Indeed this is your best bet, a minor rod of widen would only cost 14000, a great deal at your level, but well worth the trouble for the meta magic feat, as the feat itself is not worth taking IMO.
Also I would ask your GM if you could retrain gifted adept, as next level it will only be adding to spell penetration. unless intensify spell, and then it will only be relevant for another 5 levels and then again only useful for spell resistance after that.