I used internet stuff exclusively for preschool, we made great use of the library, and had a great year. Lots of fun (but he was doing Kindergarten work mostly).
Kindergarten, I looked at what the public schools were doing (scope and sequence), and I bought Sonlight Core 1 because I liked it best. To help save money, I didn't purchase science (it's not really taught in our public school until 2nd/3rd grade). I also didn't buy handwriting because I was going to use all the stuff online for "practice printing" and it seems silly to purchase books.
1st grade, I bought the handwriting program. It's easier for my son to do something that is "workbook" style rather than something Mom made up. Science we're still doing "unschooling" approach mixed with unit studies (basically anything that strikes DS6's fancy we study until he's done). I believe we've hit all the major things in science for early years, and he's on to detailed information in other areas like space and whales/dolphins/sharks. Dinosaurs are the next level down (knows the meaning of the root words that make up the names for about 25 dinosaurs and why they are called that, knows skeleton structures for the main groups, understands the difference between meat and plant eaters, the body adaptions for each....and a good study on archaeology). Space, well, he knows more than most college students know about space and our solar system. We're currently learning about solar energy (he really enjoyed burning things with the magnifying glass).
One set of books you might like is "What your X Grader Needs to Know" (put grade in for the X). That is where I'm making sure DS6 knows what is considered "average" for his grade level, and then I double check with the public school scope and sequence (availalbe online).
But to fully answer your question, yes, you *could* do it all from online lesson plans. However, you would really need to watch the quality of various ones (some are much better than others). We use them for supplementation mostly. It is also MORE work than purchasing a curriculum (and no I'm not using all that Core 1 has to offer either). I know that some of the more experienced parents in our cover school use less set curriculum, but almost all of the "newbies" are encouraged to go with a complete package deal.