One of the worst blunders in my wheeling-and-dealing is that I let go of a pristine Agfa Record III with Solinar lens. It was not at all expensive at the time but now it is getting appreciated by users, so prices are on the up, but still below that of comparable Super Ikontas etc. It has an uncoupled rangefinder (no big deal for me) and the Solinar lens is first-class, comparable (and at times superior to) the famed Zeiss Tessar, although most are found with lesser optics. I still haven't forgiven myself for selling it.
Another often-forgotten marque is the Ensign, from the UK; the Selfix 820 was equipped with a variety of lenses, but go for a Ross Xpress, prices are also on the rise but the glass is worth it too.
The main thing about folding cameras is that, mechanically, some of them do not fare too well over the years; even the Ikontas can suffer from the camera front getting wobbly, so it is an important thing to check.