aprox u need some what between 500-700 in my experiance,

invest in the polarised tech it doesn't take too long but i've noticed it saves a lot of ships from being alpha striked

Don't build any ships. Disband what you start with to save upkeep. Let the probe hit Earth and run the Archer chain immediately. Your Starbase can solo the superweapon. This strat lets you colonize all nearby planets, max your Starbase count, start cranking out a 10x NX fleet right away, and found the Federation by about 2165-2166.

Try it out.

You're meant to be able to pull at least the Kumari and the Enterprise into the fight, which adds about 1.7k power into the fight, and if you're lucky the Seleya and some Xindi frigates will also show up. If you want to fight off the initial probe in 2157 then you'll need to buy alloys to build up to fleet cap.