Upgrading the SSD is relative. An analogy helps here; let's compare the 750 to a Ferrari, the 950 to a Bugatti and the HDD to a Mustang. Obviously, both the Ferrari and the Bugatti leave the Mustang in the dust. However, between the Ferrari and the Bugatti there's much less of a difference. Both give you great performance, but both will perform the same if on a highway with speed limits, which is where they'll be used the most. Only under special race conditions will the Bugatti outperform the Ferrari.
It's the same for SSDs. Between SSDs and HDDs there's a huge difference. It's like night and day. However, when it comes down to individual SSDs, there's much less of a difference. For normal tasks, it should be almost identical - a second or two slower if at all, tops. For heavy tasks like media encoding and video rendering, provided the rest of your HW is up to spec, you may see a larger difference, but this again is a relatively small compared to that between an HDD and SSD. It's up to you if that difference is worth paying for. While the 750 EVO is a budget oriented SSD, it's specs are almost identical to it's older brother, the 850 EVO. I'm typing this running on a 120GB 850 EVO, and I can tell you it's blazing fast. Booting Windows takes 15 - 20 seconds on average. and Photoshop opens in literal seconds.
Now on to the CPU. Keep in mind that these are actual Desktop CPUs that you're getting; not some throttled down version of an i5. An i7 would be beneficial in most cases if you're looking at future upgradability. Right now, even an i7 980 (Which is now a 5 year old CPU) can be used to game at relatively high settings at 1080p with the right modern GPU. However, with a 6600K and a 980M you'll be bottlenecked in the vast majority of games by the GPU. Right now, a 6700K will have almost no advantage over a 6600K in games. And seeing as you can't upgrade the graphics chip in a laptop (It's soldered on), there's really no point getting an i7. Of course if you were to use the laptop for CPU intensive, multi-threaded workloads like video rendering or running multiple VMs, then an i7 will make a case for itself. For gaming though, same some money and stick with the i5. It's overclockable in any case, so you should be able to get a slight speed boost with a mild overclock without overwhelming the laptop's cooling system.
The RAM is less of a debate. If you want get 16GB. It won't make much of a difference now, as any game will be more than happy with 8GB. However, remember that not so long ago we were on Windows XP systems with 4GB of RAM, which seemed like plenty. Now, it's barely enough. Applications are becoming more RAM intensive, and it's nice to have extra so you won't have any slowdowns later on. However, 8GB is more than enough for gaming, so if you don't want to break the bank stick with 8GB. RAM can be easily added later on if need be.