I think of Kowa as the Jaguar automobile of cameras. Kowa was a very good Japanese camera maker, but their SLRs have a bad reputation. Their Achilles Heal is the leaf shutter, which was prone to break and difficult to repair. Modern Photography had a roundup of 50 great used camera buys and 3 Dogs to Avoid, and a Kowa SLR was one of the dogs. They said it was difficult to find people who were willing to repair the shutter, and none would guarantee their work. And that was in the early 60s when the cameras were relatively new. Imagine what it's like now.
I've seen a number of Kowas for sale with broken shutters.
Kowa insisted on using leaf shutters when pretty much every other Japanese maker (and many Germans) were using focal-plane shutters. The big advantage to leaf shutters is that you get flash-sync at all speeds, whereas focal-plane shutters of the 1960s usually topped out at 1/60th (1/90th if you were lucky).
Kowa is more highly regarded among the medium-format users. The Kowa Six is, I believe, still valuable and collectable.
E: base model. Fixed lens with accessory adapters for different coverages. Selenium cell meter mounted on prism; circa 1961
H: budget version of the E model; Has an automatic exposure mode; circa 1964
SE: an E with a CdS meter that replaces the Selenium cell; circa 1965
SE R: an SE with bayonet mount interchangable lenses (Kowa proprietry mount); circa 1966
SE T: an SE with an internal through-the-lens (TTL) meter; circa 1966
SE T R: an SE T with bayonet mount interchangable lenses; circa 1967
SE TR2: SE T R with a slightly faster 50mm ƒ/1.8 lens, cosmetic tweaks; circa 1970; I believe this is the last 35mm Kowa made.
There were also some earlier models like the original Kowaflex, a model R, maybe more. I'll add them if I come up with anything useful.
So if you want a Kowa SLR, which model should you get? Whichever one works.
There's also some confusion because Kowa sold cameras and lenses under the Koma name; Komas tend to be earlier.
This SE is often described as having interchangable lenses, which isn't strictly true. The 50mm is the base lens, and there are a couple of auxiliary screw-on lenses for different focal lengths. I have two and they're both under the Koma name.
Camera manual: Orphan Cameras.com