I have a 4ch amp and 2 pairs of 6.5 speakers. I would like to give it a go at installing it myself. Shops around me want $250-350 for install =/

I still have the stock radio in my 99 ES300 and I need to know what adapter will I need to be able to run an aftermarket amp.
As far as the wiring kit goes, should I go with 0gauge or 4gauge for the amp? Can I use stock speaker wires to wire up the new speakers? If not, what gauge speaker wire should I go with?

Appreciate any and all help
-Noob