Can I use my Mid 2014 iMac as th external display with a Mac Mini M2. If yes, what cables/adapters are needed?

No none of the later Intel, M1 or M2 Mac mini's support using an iMac as a display.

However you can use most any aftermarket external display with the Mac mini's.

see > Connect a display to Mac mini - Apple Support

The last Macs that could use your Mid 2014 iMac as a monitor in Target Display Mode were 2019 models with Intel processors. Apple Silicon/M-series processors are not supported.

not using target display mode

read

Use iMac as monitor: 5 Solutions - 9to5Mac

all will likely require a real monitor connected also, even if only for configuration