Here are a few suggestions:
Launch an elevated Command prompt and enter the following commands, then restart your machine
Dism /Online /Cleanup-Image /RestoreHealth
Both commands take quite a while - 20 minutes or more (each)
If the above commands don't help, try a refresh (repair install - keeps your data) or a reset (clean install - all data lost)
Do the refresh first, then if necessary try the reset
Refresh Windows 10 - Windows 10 Forums
Reset Windows 10 - Windows 10 Forums
I don't see anything in the machine specs that would cause the issue you're seeing. The only thing that caught my attention was Windows 8, not Windows 8.1. If all else fails, you might consider putting Windows 8 back on the machine and upgrading to 8.1 - it's a pain ... I know.
Also consider dual booting instead of overwritting your primary OS.
See: Windows 10 - Dual Boot with Windows 7 or Windows 8 - Windows 10 Forums
This saves you a headache if things don't go as expected - you still have your primary OS and you can install Win10 builds when they are available.
I understand people want to test the upgrade and that's a good thing. When you have tested that path and you're not getting anywhere, give your feedback to MS so they can collect the telemetry data and then decide how you want to test Windows 10.
Scan your machine with the compatibility tool:
Windows Compatibility Center
I still suggest updating Win8 to Win8.1 (there's a link in the compatibility page above) - while not in the requirements, it does offer one advantage - upgrade through Windows Update. Win8 can only be upgraded via an ISO