There's no point in pouring any more money into that retail piece of junk except for you GPU which you can port into the new build. Build a new one, you have enough money for it, unless you are getting a new monitor. I think you may have to get a new copy of Windows OS as well, you can't port an OEM one. So your budget may be a little tighter than it is, but you still can do it.
As long as you have the parts set putting them together is real simple, it's like LEGO blocks. Watching a few YouTube videos on it should help a lot. Configuring BIOS may be a little tricky at first but even that's simplified now with UEFI, one press on default settings should get you going. Installing OS is the last thing and that's as easy and same as installing a game.
There's a first time for everything, you can do it, it's really fun. But more than anything you won't get a better performing PC at the same price buying a pre-build.