sorry, but no - the model is not "fine" - it is poorly executed.
One should not need to make special changes & jump through hoops in the slicing software to get it to slice correctly.
It should be a simple matter of load, slice, print - not load, futz around with obscure settings, then slice and hope it slices right.
If a model comes up as non-manifold in any of the common slicing programs, then that is a problem with the model itself and it is the designer that needs to fix it.
and No, you should not "need" to buy slicing software to get a model to slice correctly. If that is their solution to a problem with the model, then sorry, I wont buy their stuff.