1% aircraft can be found at avhistory.org   These are aircraft that are supposed to be within percent of how they would fly, handle, behave in the real world. In order to use them you have to install several other add on available on their site. Also, many of the new missions require these 1% aircraft. Adding them is quite easy. If I rememeber right, they are self installing.

As for essential add ons, there are some new FX packs out there which can really enhance the game. At www.combatfs.com they have an array of great stuff. Also they have links to point you in the way of virtual squadrons.