for what they're worth. Assuming that this leakage is as rare as you say it is (5 years is a pretty good span of time) on the side of the basement you intend to refinish, I would suggest the following:
Put 2x4 "sleepers" of treated lumber flat on the concrete floor and run them so that any future leakage that might come up in the finished side would run along the floor between the sleepers like a channeling system and over to the unfinished side of the basement, where I assume you already have some type of sump system to take care of the leakage that's already occurring. Put a moisture barrier over the top of the sleepers (stretch taught and stapled) and then put your 3/4 plywood subfloor over top of that.
Granted, this is not an ideal fix, and will raise the floor of the basement by 1 1/2 inches for the 2x4 sleeper plus the 3/4 plywood subfloor, but it will solve the problem of wet flooring or wall panels should any future "rare" leakage occur. You will probably also have to reframe the connecting wall between the new dry area with the normally damp area so that the sill plate rests on the sleepers and allows the water to pass beneath it to the room on the other side.
Of course, if the leakage decides to become more frequent than every 5 years, you will still have a major moisture problem in that area and you will probably also be able to add mildew and mold issues to the list. But this will definitely be less expensive and more do-it-yourself.
PS: In addition to perimeter drains and other expensive stuff, have you looked into epoxy injections? Did this to my mother's home in one of those rarely-leaking-type-cracks and it worked great. Not very expensive, either. Just another thought.