The SSD is the biggest upgrade you can give yourself. Don't settle for a spinning platter hard disk.
The cpu speed differences between the ix-4k series and the ix-7 or 8k series aren't as big as the performance difference by having 4 cores not 2 cores. At least some of the IT guy recs are only 2 core -> less power for more money. Hard pass.
I was looking at selling a ~2 year old razer blade 15" (before it got repurposed to the kid's computer). It was something of an experiment for gaming with their tb3 external enclosure. The experience wasn't fully satisfying. But when I was looking at pricing, ~500 was the right ballpark. The specs of the 2017 blade 15 (gtx 970, high end i7 quad core cpu, 16gb ram, 256 gig ssd disk, touchscreen, high res display (3200x1800 I think), fancy black brushed aluminum case, etc.
As for where? Craigslist?