You can either re-use the same buffers for each FFT (often you need to copy data to/from the in/out buffers and perhaps convert between integer and floating point anyway) or you can use one of the "many" FFTW plans from the advanced interface, using e.g. fftw_plan_many_dft().
For an inverse FFT you probably want a C2R plan using either of the above methods and use the FFTW_BACKWARD
flag when creating the plan.