I've never bought Legos, but it seems that I may have bought most of the Potato Head parts (fun for my youngest AND me). My older girl is ready for Legos though. I don't mind the kits but like "free builds" just as well if not better. Are the cups they sell - the ones you can fill yourself - a good deal? Like do they work out cheaper than buying a couple kits? I have no idea if they're worth it or not.

thanks.
The pieces for the cups at the DTD
Lego store are great in addition to sets you already have. Like clock pieces or steering wheels or wheels. They aren't typical bricks, though.
There are building sets that you can buy, usually from
amazon or the online lego store. I rarely see them at a brick&mortar Lego store. Buy two or so of those sets and it gives you lots of basic pieces.
But here's the thing...Lego is Lego is Lego, whether or not there were instructions in the box. The pieces for a Star Wars thing can be used for a house or a car or a rocketship. My son has a ton of Legos (it's an addiction for us all), most of them are from sets with instructions, but right now (while waiting for me to be done with the computer so we can get back to homeschool) he's playing with a bunch of pieces, making stuff from his imagination, and not minding at all where the pieces came from.
The age ranges on the boxes are based on frustration level (once you're past the kid-putting-parts-in-their-mouths-and-choking stage, that is...during that stage, they get Duplo). If your child is good at being patient, if you're willing to help them learn to read the instructions and help with building sometimes, you can go up in age. I feel that Legos have helped DS in many ways, and reading/following instructions is a big way.