Hey Mohamed,
Fab that you are going to edit a feature film, tough times ahead, mixed with excitement, and happiness when you reach the other side of that experience.
You are not going to get far with a $500 PC.
Just stepping back, why are you using Premiere Pro for the job?
Why not use the free version of Davinci or low-cost Vegas or similar, where you are not paying a monthly subscription fee? The money saved might help you finance a better PC or Mac to work on. And, PPro is just not stable enough to do a feature film on IMHO and from recent experience.
Obviously you need something with enough power, memory and drive space that can run the software, without lag as you suggest.
“Also, Heavy smooth browsing (+100 tabs) for researching my scripts and will be reliable for 3-4 years with no lag or freezing”
Just forget about using your main editing system as a distraction for browsing and searching on the internet – even a 100+ tabs will eat your memory and slow down your project. Instead get a cheap tablet or Chrome book or old Mac Mini, but under NO circumstances run that kind of folly whilst offline editing a feature film on a minimum spec system – sorry for being blunt about it, but I too am one of those who will never think twice about running 1,000+ tabs in my web-browser. Do NOT do it whilst editing.
The lag itself is mainly going to come down to the size of your files and the speed of your media drives – SSD is preferential for editing and HDD can be used for back-up.
Off-line = proxy files, or down converted files for the purpose of working on a lower speed system and be able to store more.
Before deciding what to do, run a test for what proxy file format works best for you, and if you can do an online from the data at the end of the project?
Hope this helps get you started?
Atb
Mads