Which shop(s)?
Regarding the questions:
1. You know about clock speeds and cores, right? Higher clock speeds are always better, more cores only work if a program can actually run on that many cores. E.g. TF2 doesn't gain anything beyond 3 cores (and not much beyond 2) because it can only use 2 but then there's obviously still the OS and GPU driver. Rendering/Streaming will use all the cores you throw at it. Due to how CPUs work (I won't go into it) no core is ever perfectly utilized, so what happens on an i7 it reports 8 instead of 4 cores to the OS to get a bit better utilization (most say around 30%). So when streaming it would perform like a 5 or 6 core, but nowhere near an actual 8 core, and for anything that uses 4 or less threads (like 99.9% of all games) it won't make any difference.
2. Depends on how much space you want/need. A 250GB SSD will be cheaper than a 120GB SSD + an HDD.
3. Basically if you get >120 fps G-Sync/FreeSync become useless, if you get <120 fps Lightboost becomes useless.
For Lightboost the XL2411Z is the best options.