The Bentusi ship will leave but I don't dare risking not having the Ion canon for the rest of the campaign, I think that sierra maybe have done so that you will get the tech later on to be able to build the other ships which requires this tech but they maybe relied on that people would buy the tech and therefor maybe haven't given people a second chance.
All this happened on the fourth mission and I wonder if anyone that dares to take this risk will reply here, that way we all will share what you know about this, in the end 500 resources could actually be what stands between adding, completing or building a few ships depending on what.
I believe someone on this board tried it many years ago, next time you see the Bentusi trade barge you can buy it too.
You see it again about 5 or 6 missions later. Difficult, but doable without cannons.