Good working F's can usually be had for cheap. They won't be pretty looking, but, nothing that a set of small screwdrivers and spanner wrenches can't fix.
I'd considered getting mine overhauled to regain the use of the mirror lockup feature, but, then a friend offered his body, which is in nicer shape, for less than the overhaul.
With respect to tensioning the film, that's something mentioned in the instruction book anyway. Supposed to take up the slack when loading the camera, then verify that the rewind crank is turning when film is advanced.
-J