Have you already established a clientele? Or are you able to at least anticipate what general colouring the majority of your clients will fall under? The reason I'm asking is because if your clients at this time are all in the same general category, then you don't need to invest in a ton of foundation shades at this time, you can buy 2 shades and mix together as needed.
For example, I freelance as well (just starting out too!). I know that my clients right now fall mostly in the C/NC categories (Yellow-Golden Olive/Golden Beige). So, to start, I bought two foundations in the NC category. One in a light shade and one in a dark shade (NC20 & NC50). I haven't had a problem matching the right shade yet! Even with myself, I'm an NC30, but when I mix equal parts of NC20 and NC50, I get a PERFECT NC30 match.
In terms of powder, I suppose you could do something similar, but I like to use a translucent, colourless powder to set. That way, I don't have to worry about matching. Or you could use MAC's Blot Powder to set, since those come in Light, Medium, Medium/Dark, Dark, etc. shades so there's less decisions to be made in terms of colour spectrum, like there would be with the regular pressed/loose powders.
Are you currently working somewhere as a freelancer right now? It sounds like you're working with a modelling agency of some sort (if so, lucky you!! just starting out and you already have an awesome job!) If that's the case, where you'll be working with lots of different models with different colourings, then you may have to purchase more in the other shade categories. Get yourself a MAC PPID card to save yourself some money : )
Hope this helps! Best of luck to you : )