Cooling solutions for a SFF build.

[Abs_McBain]

I bought a Small Form Factor PC second hand which only had a CoolerMaster AIO fitted to the CPU, with no additional intake/exaust fan fitted on the case.

While it seems to be fine for casual use, I don't quite like the idea of just one fan in there. Would it be best to put in a fan on the rear or side as an exaust or intake?
Also this CoolerMaster AIO tubing is quite long and cramped up in the case. Would it be wise to replace it with a small Noctua heatsink, such as a NH-L9x65? CPU is a i7 8700, with a TDP of 65w - according to Intel's spec sheets.