I have a fondness for Heliar and Novar lenses. A Bessa RF w/ a Heliar (or even a Skopar) would yield superb 6x9 negs. Or get one of the Ikontas w/ Novar lens. Actually, there are a variety of 6x9 folders that will give you excellent shots. Film flatness is not an issue w/ a red window camera. Just don't advance the film all the way after each shot, and snug it up the rest of the way after you've unfolded the camera. A service is a DIY thing. Clean the shutter w/ lighter fluid if it's sticking, clean the lens elements, and confirm and adjust the focus at infinity w/ a GG on the frame rails and a loupe. Very simple stuff.
Save yourself a LOT of grief and buy an inexpensive electronic shutter tester that runs on the free program Audacity. These testers can be had for under $40, or you can build one for peanuts. It's imperative to know at what speed the shutter is firing at, vs what it says on the shutter. A nice clean will nearly always get them up to spec (usually a stop slow), and if your tester shows that you're experiencing shutter bounce, it's better to sell the camera and start anew, as this is a difficult issue to remedy. Otherwise, even a nice under $100 6x9 folder will give you wonderful photos. Unit focus is better than front cell only if you're shooting portraits or up close.