Everything is going well now at this property but I think central air and copper pipes should probably be avoided in certain neighborhoods.

If you're in the Air Force, the portal has a link to Universal class where you can take courses on property management and business.

I think my houses are nice, but very basic (refinished hardwood floors, central heat & air, white appliances, sometimes but not always new fixtures, laminate counters), and my rents edge just above average.

have not had to do this, but why not try and call an appliance repair place or air conditioning contractor and see if they can remove the refrigerant ?

Air-born asbestos is very dangerous/ toxic and its not safe to have people in the home while this work is being done. 

I have a 2 bedroom 1 bath  , 800 sq foot bungalow  for $35k that I am rehabbing to rent and eventually sell in 10 years (ARV $63k)Should I go with central heat and Air, as it would payoff for a resale later or a less expensive route such as a PTAC unit. 

@RickWell, your assumptions are mostly correct, but a NPN can go pooof into thin air and be worthless.

Just letting a little air out where you may have gotten pumped up a bit too much. :)  

Additionally, the white membranes significantly cut air conditioning costs in the summer. 

We are landlocked and surrounded by three private universities and an Air Force base, which makes us a unique market.