I upgraded from win 7 professional to win 10 pro today. Officially drivers for 10 for my laptop 5520 15r doesn't exist but everything works and no unrecognised hardware. Probably adopted from win 7. Only problem i had when i selected updates for windows 7 that update center dissected. So i was in endless loop of retarting restoring and failing updates. So i restored my win 7 and let update center select updates. Updated and restarted few tinemes then i upgraded to 10. Scared about drivers i used option to to keep files and apps (i only had chrome and img burn). After few restarts my windows 10 booted without any issues. Win 10 folder was about 20 gb and win.old folder(win7) about 30 wich means that win 10 needs less space. About resources as i can see 10 is slightly faster than my restored win 7 was. So i think you can upgrade just dont delete windows.old folder cause you need it to be able to downgrade in first month if needed.
