I have used almost all of the SL cores. (We didn't use pre-school because it didn't exist back then, and we did the combined world history called Alt. 7 instead of 2-year version). I now have two students in high school who are working on a combination of Cores 100, 200 and 400.
It took me a little while to convince myself to buy the cores back when I first started. I also have a Master's in Elem. Ed. and figured "I can do this on my own". I kept drooling over the other cores that were a bit beyond our reach at the time and figured I would just wait and begin with those. After 6 months of doing it on my own, I caved and bought SL K (sorry I can't seem to translate to the new core names). I have not regretted it for ONE MINUTE. I have loved reading to my girls, we have learned so much together, and I am amazed at how much they have retained from this curriculum. I would not hesitate one bit to buy the next core and move ahead. If you get to the point where your child's age/ability are not ready for the next core, you could take some time for other studies. (We did break a couple times to do KONOS unit studies.) If money is the issue, you could probably find used cores or just by the IG and gather the books on your own. I know that many SL users do that. I, on the other hand, still have every single SL book on my shelves at home. It's quite a collection. (Ok- there is one book that I let someone borrow and I haven't seen it in several years.)
I guess I should have looked ahead to see if you had responded since your original post. I hope you find what works best for you.