This Hardware Development Kit (HDK) for Arduino Uno is your easiest way to get started designing an Arduino Uno project.
The guidelines give you the drawings of common circuits like power (voltage regulator), Oscillator and Reset, etc., which will greatly shorten the designing process.
This kit also includes commonly used components like Atmega328p, 16M crystal, resistor, switch, etc. And the quantity is sufficient for at least 2 prototypes, which can get you out of the headache of purchasing small quantity components.
The parts are all in DIP which can be assembled by hand without much difficulty.
Most commonly-used components
Available for individual purchase
Includes DIP Atmega328p with Arduino Uno Bootloader
IC Pinout configuration diagram
Modular circuits breakdown, e.g. UART, Power, Microcontroller