If the fan clutch is original, there is likely no way that it is functioning 100%. I would start planning on replacing that even if you need to add refrigerant. When you do get readings on the gauges, note them, the ambient temp at the time and whether the compressor clutch is cycling and what the gauge readings change to when this happens and then post that info here. You can also go to you tube and watch several videos about what to expect. I say watch several because they are not always the same.