I remember
@VirtualLarry bought that PC as well. He might have some good advice for you.
I bought the HP "Power Gaming" PC, from Walmart early in Dec., but it was an advertised Black Friday special, for $500 + tax in-store.
I haven't done too much with that PC. I added an SSD, first-off, and unplugged the HDD, and I've used it for Folding@Home (300K-330K PPD), and currently for NiceHash ($2-$4/day). I haven't actually tried any gaming or streaming or game-streaming on it.
I too was surprised to see a 300W Lite-On PSU, WITH a 6-pin PCI-E connector coming off of it.
I honestly don't think that I would bother to upgrade that PC much, though, besides adding an SSD (there was a spare SATA port on mobo, and a spare drive bay).
Better off getting a Ryzen (or wait for the newer 2000-series) CPU. My main PC is a Ryzen R5 1600, and it really rocks. I was a somewhat early-adopter. I'm glad I did, though, with all of this "Meltdown / Spectre" garbage affecting the Intels. (AMD Ryzen rigs aren't affected by Meltdown, and only a tiny bit affected by Spectre.)
For that reason alone, I might suggest getting a Ryzen rig, but honestly, this thing does everything that I throw at it with aplomb.
For gaming + streaming, though, I would strongly suggest spending a little bit more, and getting a Ryzen 7 CPU, like the 1700X or 1800X.