A guy has it for sale and said it's only 4 years old. It's a 3.5 ton. He said it ran on propane but will also run on natural gas, which is what I need. First off, how old is that model number and is it a quality unit? And will I need to change anything out for it to run natural gas?

I have an old Westinghouse package unit that's been in operation since 1966. It still works but I want to update to a newer more efficient unit without breaking the bank. Is the York model number DINA036N07206C a quality unit?

And will i need to change anything out for it to run natural gas?

Yes a professional will need to change it over with a kit for that unit containing all new orifices for nat gas. Special tools and knowledge are needed to do the conversion. In other words it is not a bolt on job.

3 ton unit. The year is in the serial number