Apply directly to Airserv since you meet their minimum requirements. They don't as such fly for the UN so much as for NGOs to the best of my knowledge.
Some SA registered and some N registered A/C.
Validation to SA licence is no problem but, you can only fly types you're rated on on your foreign licence and no types can be added to your SA license.
Executive Turbine amongst others, fly King Air 200s. With your time you should be able to find a job.
GB