On my second year learning about 6x7 and 6x9, so far I have 3 cameras and the 4th is on the way.
Used prices are falling away.
Film Flatness is way down on my list of items I have to improve as a hobbiest and probably not even an issue.

Higher on the list for purchasing old equipment - shutter cleaning and calibration, and deteriorated light seals.

As far as focussing the folders, the issue is that modern films, say iso 200+, will force the thing to be ~ f/16 if outdoors; I found scale focus gives sharp results.
If you are going to do close ups / indoors, maybe you need an slr or (an older) camera with ground-glass focussing.