There's nothing wrong with plastic on the outside. However, plastic on the outside often meant plastic on the inside, often in the gearing. The industry worked hard to cut production costs and maximize profits, and the use of plastic in cameras helped to achieve this. Sometimes it worked. Sometimes it didn't.
Most of the Japanese consumer models in the 1980s up through 2000 are made with lightweight parts. The pro models are different. Nearly all Japanese cameras since the 1960s use foam seals that will need to be replaced if the camera is 20 years old or older.
The Nikon FM10 is made by Cosina and mostly likely is made for lightweight use. A fine camera, but no better or worse than its contemporaries.
In K-mount, the 1970s models (K1000, K2, KX, MX) are excellent cameras.
I've used a Nikon FE, FM, FT3, F2A and EL2. All well made.
The Yashica/Kyocera Contax are excellent cameras with Carl Zeiss lenses that have an excellent reputation. Also, the focusing screens in the Contax cameras tend to be brighter than other cameras.
I've been working with a Minolta XD-11. Excellent camera. Very sturdy. Small camera.
The Olympus OM1, OM2 and others (not the OM10) are small cameras, mostly metal and have a large following. The single-digit OMs have their shutter speed dial at the base of the lens mount.