That�s a cool offer. 20 turns of peace does sound like a good deal for 100 ships. You should have plenty of time to either get on his good side or to rebuild your fleet. Remember; in 20 turns he would have paid approx. 3 times the initial value of the ships in maintenance if he decides to save them for use against you. Of course; If you have other enemies, you gonna get smoked
Going down in flames is also a cool option.
You already know my opinion of plan B