And remember: Excel (or any other spreadsheet) is your friend. Keep a list of all the pieces you need (extract from BL), and keep track of what you've purchased from where and how many (plus cost + shipping if you want to find out how much you saved versus just buying a used one). BTW, unless you're really strict on yourself, you might find you get close to what you would have paid if you bought a new one ...
And the obligatory warnings: You will most likely run across incomplete orders (i.e. you ordered quantity X from a BL seller, but got X-1, or got a different part or something like that). Plan for this - unless the part is really expensive, just buy a few more than you need (if the seller has them available).
And you will inevitably go hunting for that cat, or that lady minifig, or those sand green bricks with groove etc. Resist the temptation, you must.