Yeah don’t get lost, its mind blowing what you have the potential of game wise, but just get the campaign under way and def get SU (six updater), it will give you the servers and mods needed for those servers, taking away the headache. If you have watched youtube MP, especially groups/clans, then you can see the great potential..
A good idea to have when and if you start modding the game is to back-up onto a external hdd or similar. I back-up my Arma 2 (I run Arma 2 Combined Operations, which is your version with Reinforcements added ( BAF British Armed Forces, & PMC Private Military Company), on a hdd drive, plus a separate one for the mods (seen in the links below). However you only use certain mod set-ups for different types of game play. I mostly make & play missions for a small group of players on a private server, I have listed below screens shots of mods I have, these don’t include many Arma (1) islands as they are on a separate hdd, but this gives you an idea of the amount of mods that the community have produced. I use Six Updater (SU) for some mods but also keep others, not available on SU, upto date manually. But SU is the way to go at the beginning, it sets it all up for you.. Obviously you won't use all these mods but its handy having as many as pos so that you can experiment as to how they change game behaviour/game-play..
Screen shot 1
Screen shot 2
Screen shot 3
I am, and I dare say many will be, more than happy to point you in the direction of tips, mods, islands etc if you need it. Sound wise one main mod will be the best to begin with, I recommend as already said in the thread here by a poster, ‘JSRS’ it is possibly the best overall sound mod, but there are others that have certain sounds that are better for certain items, but you can if you know what your doing mix and match sound mods, but that would be for when you are well into learning about the game..
Arma 2, with all the additional bits i.e. Operation Arrowhead, Reinforcements (BAF, PMC), etc is a huge game, then you have the community mods/addons that add upto over 400 with the islands, so the game-play potential is massive, the more you learn the better it becomes..
Play the campaign, then have a look through the editor, this is where, in my opinion, Arma 2 wins out, you can design and make your own missions, which can take from just a few minutes to set-up, to a few weeks, depending on how complex & difficult you want your missions to be, but great game-play can come from even the smallest skirmish type missions that has only took a few mins to set-up.
There are clans and groups (my own included), that play Arma 2 more as a sim than a game, that’s where Arma 2 takes to another level, by far the best of its type available..