Do you need another camera? If you do, will this camera do what you want and provide you with good value for the money? Bear in mind that a clean used FE2 will do 99.7% of what an FM3a will.
As for buying it as an investment, you could easily make money on the camera by keeping it in perfect condition in the box somewhere cool and dry, but you can average 20% in aggressive mutual funds. Buying camera gear as an investment is always a gamble. Cameras are tools, not investments, in my opinion, although good cameras hold their value well if you take care of them while you use them.
I like cameras and I have far too many, so I'm not really the one to ask. Having a second SLR is not at all overkill for many reasons, which I'm sure have been discussed ad nauseum on this site among others. The question is this: is this the best way you can spend $559 to improve your photography? If so, buy the camera. If not, buy the alternate camera, or the film, or the lens, or the combination of those three, that is the best way.