“Fixed fortifications are monuments to man's stupidity.”
I have taken a different view of preparedness. A good rifle and a means of mobility are worth far more to me than a fortress with a stocked larder.
The first step is to get as far away from other people as possible, avoid the chaos. The problem with a fixed site is that is becomes a locus of interest amongst other survivors. It is very hard to keep the details of a project as ambitious as yours a secret, and in the "post-apocalyptic" arithmatic it is far cheaper to take what is yours than it is to prepare myself. My own personal morals prevent me from taking from others, but I realize that I am probably in the minority. So my own philospophy is make a small target, rough it for a while untill things calm down, re-integrate into whatever society re-forms.
Plus, given the choice of eating MREs for 4 years or being vaporized, it's really the flip of a coin...