Everything in this plastic kit (1743-3535 or 1743-3536) will fit your 2003 KX250, EXCEPT the lower fork plastics. The lower fork plastics that come in these kits will not fit the 2003 lower forks.
This plastic kit will bolt on your 04-07 KX125-250. It does not require a new seat. This plastic kit also bolts on the 2003 KX125-250 (Minus the lower forks)
You would need to order a graphics kit that is cut for the KX Restyled plastic kit. A graphics kit that is designed for a stock 2003-2007 KX125-250 will not fit the restyled plastics.