I can't answer #1, but I can tell you right off the bad that if you have ATI you will not be able to experience as much of a fluid game as if you have NVidia. That thought is not the game's fault, but ATI's but I will try to not be mean to my card.
SS+ adds new features to the game and alters the gameplay, as well as adds new ships for pirates, and variations to ship types. If anything it enhances the gameplay and you better read on it's page what it adds.
Nexerelin changes the gameplay, from static to dynamic. I say that because you see factions fight, but they actually can take over planets and that means some can grow large and some can die. That includes you, since you can do that for another faction, or even take over planets for your own faction.
Unlike SS+ and Nexerelin that can play along other mods, mods like Ironclads can't work alongside faction mods. Although Ironclad has a specific mod that works with it (the VNS ones), it can only work by itself. Mods like Ironclads are called TC or Total Conversion mods and they can alter the way the game/galaxy works. Ironclads makes the game (imo) feel more realistic and MORE hard. Definitely when I played, I felt I was playing kind of a different game.
It not not good to get "all" of something in general. It is good to read what each mod does and decide if you want it or not. Personally I use most, but I am just saying it is better to know what you add.
If you want a really balanced game, be it you play vanilla kind of game or Nexerelin (since nexerelin changes the way the game plays) you can choose to play ONLY with the mods listed on the SS+ page. SS+ balances the game and the mods on it's page try to follow the balanced rules that SS+ adds. So if you play with SS+ and these mods, you will have quite a balanced game.
As for updates, that is kind of impossible to answer. It depends not only from the complexity that the update brings, but also how much the creator has time to update. If anything I will say this. I too am a new player, with my first vanilla game being 0.7.1a. I have thoguh went backwards after playing the game a bit when I first started and played a hugely modded game on 0.65.2a. Sure the gameplay is different for 0.7+, but that does not mean the game was not good in previous versions. Actually the fact the gameplay changes makes it good to play even if you go even more back. I have currently gathered 0.6.2a and all the mods that existed back there and I am split between a game with the 0.7.1a mods or 0.6.2a. I am going backwards not so much for the different game play, but to experience mods that are lost in time and maybe a slightly more crude version of the game.
Have fun with the game and enjoy. I hope you enjoy this as much as I have o/