While it's often wise to purchase expensive overprinted stamps conditional on obtaining expertization, such caution is unnecessary in this case because the basic non-overprinted set is also expensive.
Papua New Guinea is richly endowed with natural resources, but exploitation has been hampered by the rugged terrain and the high cost of developing infrastructure. Agriculture provides a subsistence livelihood for most of the population of about 7 million. Annual GDP growth has increased dramatically over the last 5 years, from 1% in 2005 to about 7% in 2009. Still, the majority of the population is extremely poor, and I do not foresee the development of a significant collecting population within the country for some time.
Most of the collectors of Papua New Guinea are British Commonwealth collectors or Australians, because the country was administered by Australia until 1975 and maintains close ties with that nation. I recommend purchase of the better stamps of Papua, New Guinea, and Papua New Guinea based on the probable growth in interest among Australian collectors and collectors of British Commonwealth.
Those interested in learning more about investing in stamps are encouraged to read the Philatelic Investment Guide ($5), available on Kindle, and accessible from any computer.