I have an oil furnace with CAC that needs to be replaced in a 30 year old house. Keep the Oil system or replace with Gas? What Brands for each?

You could ask any number of people for their opinions on this subject and perhaps get a different response from each. If it were my home, I would not hesitate to make the switch to gas. From my experience with them, fuel oil heating units are high-maintenance, troublesome, and dirty. If you already have natural gas service available, a new gas furnace might even be less expensive than a replacement fuel oil furnace. Consider a 2-stage model-they are capable of operating at a lower input in milder heating weather, then staging up when the higher input is required. If by 'gas' you were referring to LP, there will be considerable costs associated with the installation of a tank & supply line. Do some research online to inform yourself and make the choice that is right for you.

Part of your research should include local contractors and heating oil/propane suppliers.