For serious use of Adobe Premiere Pro and especially After Effects, 16 Gb is a minimum, dedicated quadro or FirePro highly recommended, or buying at least GTX 1070. Trust me, I've worked in video editing field for a decade, After Effects and 8 Gb of ram is a no go. You will experience constant stutters. I worked on video editing workstation cheaper than 2000 € only when my primary system broke down and it was a nightmare every single time, especially when catching deadlines. How could you build new video editing PC for 500 $ is beyond me, especially in times when HD video is a norm.
You should look for an old workstation like HP Z240, but will still cost you around 700 $. It has Intel Xeon E5-1650 v2 @ 3.50GHz (12.660 CPU mark -> on pair with Ryzen 5 1600/i7 7700K), 16 GB of ram, that should be enough, but I would add SSD to run Windows and Adobe apps on it and maybe some newer GPU, GTX 1060 as a bear minimum comes to my mind, if you really can't squeeze for pro card or gtx 1070 (Adobe apps love NVidia GPUs). That's a 900-1.000 $ build, very cheap for video editing machine, but still useful for essential work.
https://www.itsco.de/workstation-hp-z420-8-dimm-intel-six-core-xeon-e5-1650-v2-6x-3-5ghz-22804.html