Hello, good evening, given your PC configuration, I recommend that you install the GTAV Legacy version and not the GTAV Enhanced version. and to answer your 2nd question the GTAV Enhanced version does not currently work with Fivem. The only Fivem addition to GTA V Enhanced is this one Coming Soon - New Official Modding Utility - FiveM Support For Newly Announced Grand Theft Auto V Upgrade Coming Soon but I don’t know more for the moment.
Hope this helped you 
YukomiFPS