Tom - this is a lot of stuff to buy. However, its important to remember you don't have to buy it all at once (as you suggested).

I always tell people to establish a "prep budget" wherein they put back "x dollars" (they have to decide what they can afford) per paycheck.

Use this money wisely by selecting gear that "rounds out" your BOB. For example, I imagine everyone reading this has a Scrap Yard knife...so I'd recommend picking up a LMF Army Swedish Fire Steel ($15) and USGI Canteen Cup ($5) first. After that perhaps a Katadyn Hiker Filter ($65) or USGI Poncho ($30) and 200 feet of 550 Cord ($12).

What's important:

1) You're doing SOMETHING.
2) You're creating rounded gear. Don't complete one area and have nothing in another.

Even when budget doesn't allow you can build on your knowledge and skills. Go out in the yard and build a fire with your firesteel. Practice setting traps. Whatever - just do something.