Looks like you have made your choice for some good reasons. But if you still have any doubts, you might want to check out the Fujica ST901. It doesn't get any love here at RFF, but is a good capable camera. Your criteria:
Must haves:
- aperture priority (shutter priority might be ok as well) - Has that from EV-3 to EV 18 at ISO 100.
- manual exposure mode - Manual mode from 1/60 to 1/1000 and B. Manual mode is mechanical, not dependent on batteries.
- easily accessible AE lock function - If you mean locking shutter release (which ativates the meter) so you can't run down batteries, it has that. If you mean exposure lock, no, it doesn't have that.
- price: body + 50mm (f1.7-2.0) standard lens < 50€ - I have no idea what they sell for in Europe.
- all metal black body - Hard to find one that isn't black body, but they do exist.
- hefty construction, 'classic look' (thinking of the fe or om again) - They sure do have that.
Nice to have:
- no mercury cells - Use a 544 battery, no mercury.
- shutter speed display in finder - Has that with led. Unlike most, for 1/1000 you get an actual 1000 by led, or for 1/30, you get 30 by led. So even in dim light, you know for sure what you are getting for a shutter speed. There is also an aperture reading with Fujica lenses, but it it not lighted.
- rather fe size than om size (I've got huge hands) - I don't know the size of the FE, but it is larger by a bit than an OM 10 I have.
Another advantage is that it is M42 mount, and will take any M42 mount unless there is a mechanical problem, such as some Mamiya lenses. You can use stop down aperture metering with the depth of field control, and lock it in DOF mode for ease of use.
As I said, they don't get any love here in RFF, but they are a good and durable camera. I have had my original one for a couple of years shy of 40. I have had no problems with it. The Fujinon lenses are a little expensive, but always worth every penny.
Probably even more features would be the Yashica FX 103. You might want to check it out, but I think the older M42 Yashica lenses are a little better, and the great Contax lenses sound like out of your budget.
Neither of the above are Nikons, but imho worth a look at least.