To answer the question regarding the Brutes’ preparedness, you have to look in a few places to find specifics.
From Broken Circle…
"Truth has apparently sent special orders to Brutes stationed around Earth. Why would he be giving such orders, from here? That task is below him. And orders only for the Jiralhanae? He’s also sent another fleet to Earth—again, not a job for his high station. Why? And according to data I’ve acquired from comptroller files, he has already placed a large order with the Sacred Promissory—the weapons armory here on High Charity—but he has kept this quiet even now. The order, as I understand it, is the mass production of traditional Brute weaponry. It appears to me at least that he is arming the Brutes to a perilous degree."From Ten Twenty documentary…
Truth swiftly commissioned a massive fleet and privately summoned over five hundred warships to a command-and-control station called Unyielding Hierophant. When UNSC forces uncovered this plot during the events of Halo: First Strike, they initiated a covert assault of the station. Led by the Master Chief, a strike team of SPARTAN-II super-soldiers infiltrated Unyielding Hierophant and destroyed it, along with the vast majority of ships in its charge.
The station’s ultimate goal and its eventual destruction were suppressed by the Prophet of Truth—Unyielding Hierophant’s original purpose has since remained a closely guarded secret.
Well before even that day, Truth had begun to doubt the reliability and loyalty of the Elites, who were, at that time, the heart and head of the Covenant military. Though Regret’s ineffective invasion of the human home world was problematic, Truth would use it to his advantage. Not only was the support and excavation fleet he had already sent to Earth commanded exclusively by Brutes, but he would also leverage the abandoned Brutes who had served in Regret’s initial fleet.
Aboard those vessels, the Brutes were ready and willing to unflinchingly obey the Prophet’s commands. Once Truth gave the word, these soldiers forcefully overthrew the Elites and assumed complete control of the battle above Earth. Upon further orders from Truth, they aggressively deployed significant infantry directly into New Mombasa and picked up where Regret had left off while holding the human forces at bay until Truth’s reinforcements arrived.
It sounds like Truth had the displacement of the Elites already set for commencement prior to Earth being invaded. Who knows how far back the conspiracy goes. Truth had already gotten a taste of the paradox with Elites’ psychology when it comes to honour and incredulity versus loyalty to the Covenant when Zhar tried to assassinate him and Regret back almost 20 years ago.
A secretive contingent of Brutes, under the direct command of Truth, were likely already being outfitted with mass produced power armour some time either during or immediately after the battle on Alpha Halo. We see Brutes with power armour on back during the time of Harvest so we know they were in circulation and being used. If I had to guess, Sesa 'Refumee’s little rebellion on the gas mine spurred Truth into going ahead with his plan since it was his worst fears becoming a reality. Combine that with the destruction of Halo – which they laid at the feet of Thel, an Elite – and you have a recipe for one paranoid Prophet who’s looking to make changes to the Covenant system.