Not sure what you mean by addon packs uless you are talking about service packs for a specific system.
Which sim depends on what you want to do and to some degree what your system specs are. FS2004 is the latest relase of flight simulator and needs a strong system to get all the bells & whistles from it but by no means does it mean that you can not enjoy it on a smaller system as long as it meets the min. system requirements. But plan on a full install as a partial install really slows down everything when it has to read scenery from the cd. A good quality AGP 3d card is also advisable. As far as a joy stick goes it depends on what you want. I currently use the sidewinder it has rudder throttle and a hat switch which comes in handy for spot plane views. 8)
AS far as addons it depends on how much hard drive space you have. There are a ton of addons for booth airplanes and scenery. One word of caution if you plan on adding aircraft or scenery make a seperate folder to download them into do not download into fs2004 it can cause headaches later. 8)