I got the folder fever and I’ve acquired a motley collection of Isolettes and Speedex’s and other folder brands in last couple of years. They all had frozen helicals, sluggish shutters and bellows issues. But the only folders I’ve acquired that had good bellows, snappy shutters, and free focusing helicals were Adoxes - a Golf and a Sport. Based on those finds, I would suggest you look for a folder other than an Isolette/Speedex. I think you would stand a better chance of getting a camera that still focuses and might have a decent bellows.
Stan