a) Kiddos, ssd is better in speed that one HDD, by that when you wanna "Give a second life" to a old machine, dont care if is Sata 1, Sata 2 or Sata 3, one ssd make that all your PC run more faster, example, I give a second chance to a old 2011 Notebook HP - G42 362la with sata 2, core 13 1° generation and DDR3 1333 with an not very expensive SSD ADATA Su 630 - 480 GB and run decent now
B) One Graphic card always will make your PC more Powerful, why? why one Graphic card take all the video JOB and not more your processor!, this mean that is like your pc have 2 processors, one for Video (GPU) and one for precess (CPU), this mean that you can do your pc more faster, example, I have one PC with Intel serie i7 K with an AMD R9 series graphics (Old) and I can edit video, and mix In pro tools a lot tracks in 1080p!, including play games not so old! like Mafia definitive edition!
c) I always recommended use an ssd and one dedicated graphic card and if you use the graphic card with HDMI is more faster! and you can run games more easy - Audio and video Onboard always sucks.
More Info? you can call to motha faka hardware nerd.