My main question is, for those who went through the ban before, what parts are going to be the hardest to find in the next few years?
It all depends on what is banned.
In 1994 mags that held over 10 rounds and semi-auto rifles with more than 2 evil features were banned.
From 1994 to 2004 the 5 Evil Features were:
1) Collapsible stock
2) Pistol grip
3) Removable mag
4) Bayonet lug
5) Flash hider
So most AR15's retained the pistol grip and removable mag as their evil features.
It was easy to get A15's without flash hiders, bayonet lugs, or collapsible stocks. No shortage at all on full guns, receivers, or upper kits.
If you wanted all 5 Evil Features, you had to have a pre-ban gun. Those were hard to get at a reasonable price. Receivers didn't count, it had to be assembled into a gun before the ban to count as pre-ban.
High capacity mags were very hard for civilians to get at any kind of reasonable price. Law enforcement personnel could still buy high-cap mags (marked "LE ONLY) at normal prices.