1) That is a pretty good SATA SSD, but not a lot of room for games. If you find that you are having to uninstall a lot in order to swap in a new game, then you may want to consider a drive with more capacity. Personally I use one of those 256GB Evo drives for boot and various productivity software, but I put Steam and games on a separate 1TB drive.
2) This will likely void your warranty if you still have one, so keep that in mind. Graphics cards normally should not have to have the thermal compound replaced. If you are trying to squeak out a little better cooling and performance, it may be worth a try, but you need to make sure that you do everything correctly the first time, or ells you could have problems. Video linked below:
https://www.youtube.com/watch?v=tHa7ru2_aRI
3) If you have to do it at all, just to put in a higher quality of thermal paste for performance issues, then just the once should be good. That should cover the normal life span of the card till you are ready to upgrade to something new. Most people should not worry about this though, as it tends to be something only done by hard core enthusiasts.