There is a great sonlight pre-K yahoogroup which if you join you can get a lot of btdt advice from moms. A couple of the moms on that list have also designed "add-on" curriculums that really enrich the experience, but in order to get those, you have to just join the list.
When we were homeschooling I was using the pre-K for my 5 year old (with Sonlight, the "core" years don't always correspond to regular "grade level")and I think, with some exceptions, it was pretty much on target. I did add some extra reading, handwriting, and math in myself because she just adores workbooks (go figure

). At this age, science is usually a kitchen activity, or playing with the properties of (bath) water, or talking about various aspects of our new baby (how he got here, where he was inside me, why he looks different from his sisters

, why we poop and pee and what happens to the food we eat inside us), or cleaning-related (what's a sponge? where does it come from?) etc.
We also skip all the Christian-related material because, well, we're not. We add in some enrichment from our own religion (Hebrew language, Torah readings of the week, holidays, etc.) instead, mainly on Fridays which is when we prepare for our Sabbath.
I got all of the books either second-hand or on overstock.com and I ended up paying considerably less than the core price, but then again, like I said, we skipped some. I also bought Handwriting Without Tears (great program, fun for the kids) second-hand, and the Singapore Earlybird math series, which is pretty inexpensive new. The workbooks my daughter likes I picked up at the grocery store, or from Usborne books (which is a great company I think, and I'm not a rep!).
Good luck! I personally really like the learning-through-literature approach because you can either go exactly by the Instructors' Guide and not have to worry about getting a great curriculum, or round it out/adapt it yourself and still have a wonderful spine/basic approach to rely on.