Hi, a camera that has a full manual mode can be usefull. It doesn't need to be a mechanical camera without automation. The OM1 you mentioned earlier is a purely mechanical camera. You need to set both the aperture *and* the exposure time before taking a shot. While it can be beneficial (as in battery independant and somewhat more educational) it also has some disadvantages. For starters, you *will* be slower when taking photographs. Focusing, selecting the appropriate aperture and setting the shutter speed will slow you down and you might lose some opportunities for a good shot. The OM1 also has a disadvantage that you should consider. It's light meter relied on mercury batteries. These have been banned years ago and you will either need to make a modification, or use a battery adapter.
I'd actually suggest buying an OM2n. It has aperture priority mode *and* manual mode. It uses silver oxide batteries that can be found everywhere. If your budget is very low you can actually get an OM10 in nice condition, provided that it comes with a manual adapter! These are very common cameras, but not so well built as the single digit models (OM1,2,3,4). Additionaly, the meter is not that good as OM2's is, but it doesn't mean it's not good for most purposes.
Finally, all brands you mentioned are just fine! There's no need to pick Olympus specifically. All brands have excellent lenses that complement their bodies. For a start I'd suggest a 50mm lens.