You can start with the summer uniform and get the winter one later if you want to spread out the cost.

The boys each have 2x long pants and 2x shorts. 3xshirts. 2x jumpers. Sandals with a closed toe for summer and proper shoes for winter (but they often waer their runners not school shoes). Sun hats plus a spare. I find that hats are thing that go missing the most. I have lost count of the number of school hats I have bought so now I usually send them to school in a non-uniform hat because people don't seem to keep them.

I label things in several places.