Mina-san Konnichiwa!
All the translations are good! Although the words differ slightly, they all mean the same thing.
If you want to translate using archaic(
Samurai) language, the following terms would be appropriate.
太平を望まば、戦へ備えよ : Taihei wo Nozomaba, Ikusa he Sonaeyo
The word "heiwa : 平和" was coined during the
Meiji era; prior to that, the term "taihei : 太平" was used.
The term "sensou : 戦争" first came into general use during the Meiji era; prior to that, the term "ikusa : 戦" was used.
However, while the word "ikusa" is still in use today, "taihei" is no longer commonly used, so "heiwa" conveys the meaning more clearly than "taihei."
I think "平和を欲さば、戦への備えをせよ" is better.
NANGI