The BUF signal becomes negative on falling edges. Will this be a problem for the buffer IC? And how can I avoid this?
Probably not. Most IC's have clamp diodes that clamp voltages to the lower rail or upper rail. You may not exceed the clamping current (defined in Absolute Maximum Ratings). But with 100kΩ that will not be the case.
Otherwise, you should implement clamp (Schottky) diodes yourself to protect the input.
What do I have to look for when selecting a part for the NOT gate. I read about schmitt-triggers, do I needs this? E.g. will something like SN74LVC2G14 work?
You would need a trigger point with hysteris if the signal would be very noise. I think that is not the case (when R1 and R2 are located close to the gate), so you can use normal gates.