Windows 8 -8.1?

[Exploration]

Right so recently Windows 7 broke and i couldn't find the disc or the license key to go with it. Only option i had to hand was the recovery/reinstall CD that came with my original PC. (The PC is long since modded extensively. Only original part about it is the Mobo in a completely diffrent case). Now i knew the licenses are saved in the BIOS or something for 8. So reinstalled everything and long story short, it took two fresh reinstalls to make it run worth anything. So after spending 3 days installing over 3GB of updates and it's now actually running fine.

My first question: I've gotten to the stage where the store is prompting me to run the 8.1 Upgrade from stock 8. Main question being, is it worth it or am i really leaving myself open for another busted installation and another three days of hell fixing it?. Is there any performance difference if i do opt for the 8.1 update?

My second question is, i noticed Windows Defender the standard antivirus for 8 won't actually enable the protection service. Just throws a bunch of random errors at me when i attempt to start it. So is there any Free antivirus programs which don't hog my hardware but will actually help prevent this thing getting grilled again?

No Windows 8 haters comments please, i'm well aware from this experience that 8 is a royal pain but i refuse to purchase another license for an OS which is essentially 7 years old with Windows 10 just around the corner as a free update (Meant to be). Constructive advice/Program suggestions would be much appreciated!