I’ve been considering buying a "spirit Halloween proton pack to upgrade and all for next Halloween and for my own use, what is the easiest way to upgrade and what material would I need if I buy materials here?
Buying the spirit upgrade kits made by a few members of this forum, that’s what I did and my pack looks amazing for a minimal price.
i wouldnt say its minimal 
I just finished a spirit pack for my son and did a build thread for it.
https://www.gbfans.com/forum/viewtopic.php?f=2&t=45979
I tried to include everything I used as well as some measurements and such. Could help you with deciding what to do. Best of luck to you on your pack!