If you are wearing out FM's and F100's then I am afraid that, nice as they are, Olympus cameras will probably not stand up to your usage. I have both a brace of OM2n's with a large selection of lenses and F2/F3/F4/FM2 Nikons and so have some basis for making a comparison between the systems. The OM's are quite frail compared to the Nikons - but that should be expected with the Oly's being famous for being light and compact. If you were looking for cameras for travel or hiking I can't think of much more suitable cameras than OM's.
If you already have a selection of Nikon lenses then it would make financial sense to stick with Nikon. There is also the saying "Better the Devil you know than the Devil you don't."
If you stick with Nikons I would suggest you limit yourself to lightly used professional grade Nikons.
Going by KEH prices you can pick up a Nikon F3 for $264, F4 for $349 and an F5 for $449 - all in EX condition. You should be able to get 25% or more off for a private sale unless the seller offers the same return privileges as KEH. Hardly outrageous prices.