Required components:

- a power supply (12V for the PA and 5V for the rest)

- an Arduino™-module (any model with at least 13 digital and 2 analog pins will do / in my case it's a
  rather big "Uno"-clone only because I had one in spare)

- an AD9850-based DDS module (I got mine for less then $10 incl. shipping / by the way, this fellow is
  quite noisy at certain frequencies and giving it a bit of shielding isn't a waste)

- some sort of time-source to lock on and keep the beacon synchronized / for this purpose I employ a
  DCF77-receiver which works fine throughout Europe in a not too noisy environment

- a 2x16 LCD-display (HD44780 clone)

- a PA (there are various design proposals available on the net and it's up to you how much effort you'd like
  to put into that / mine is 200mW, which I feel is sufficient for WSPR)

- an SWR-bridge, feeding into an algorithm to protect the PA (not a must but definitely helpful when
  experimenting with various antennas / it's missing in the picture above but was added later on

- last not least the antenna / if the beacon is set to operate on a number of (or even all) bands, a decent
  broadband design will be required / due to my limited space, I decided for an indoor aperiodic antenna