They aren't necessary at all though. An HDD @ 7200rpm would be plenty for a lower-budget build like his. If his budget was like 1k or more, then an SSD would be fine to add
IMO never add an SSD to your build if your budget is $700 - $600 or less so you can save the 50+ dollars that you could use on your CPU or GPU.
-snip-
modified as per your recommendations. it's like 8 bucks cheaper, but still higher in stuff
Yeah starting to look good now
If you're fine with the current pricing, then the only thing I can really recommend is the Corsair Carbide 100R for a case. IIRC it's only 39 - 49 bucks, and it comes with a load of places to put fans and radiators, has a clear side panel, and the brand is reputable as well. Also you should go with the MSI GTX 1050ti Gaming X for the GPU instead of Gigabyte. It'll match your PC's innards a bit better and is 10 bucks less.
Other than that, your build is solid