It depends what features are important to you.

As you say, you can think of a camera body as a pretty simple thing. It is just a light-tight box to hold the film (or sensor). If you go shopping for one, there are obviously many choices, and like anything, you just compare features (depending on which are important to you) and pick one that fits your budget.

First consideration for film photography is film size. Are you going to be shooting 135 film? Then narrow your choices to a 135 film body. Do you want a camera that advances the film automatically? Do you want autofocus? What metering options would you like? What exposure options do you want? Is full manual exposure OK, or do you want program exposure modes? Do you want a built-in flash? How many frames per second should the camera be able to shoot? Do you want multiple exposure capability? What about exposure compensation, and automatic bracketing?

How does the camera feel in your hands? What about robustness? What environmental extremes do you need the camera to be able to handle? Is the camera in good working condition? If it breaks, what are your repair/replacement options?

Manufacturers made camera systems, i.e. not only bodies, but lenses, flashes and other accessories. What is the availability of these items for any camera body you are considering?

Do you want to be able to use camera movements? If so, maybe 135 film photography is not your best option.

Take a look at the Nikon F6 just for comparison purposes. That's Nikon's last flagship 135 film body, and about as advanced a film camera as can be found. Have a read through the sales brochure / features / specifications and see which features look appealing. It will give you an idea of the possibilities, and then you can look for a camera within your budget that has the features that are important to you.