Whether you need a second heat/air conditioning unit when you finish your attic depends on the size/capacity of the unit you have and the size of the finished space. Your roofing materials could also have a bearing on the efficiency of your first unit. In all likelihood, the heating/cooling system you have is sized for the existing home configuration. Trying to add the attic will then overtax the system.

Another consideration is that there is no return in the attic, making air circulation difficult. Heating will be less of a problem because heat from the house below will rise. Air conditioning could be a very large challenge. The best way to keep the entire home comfortable would be with a separate unit. A heat pump should work very well. You will need less heating capacity from the unit and more air conditioning capacity. This is exactly how heat pumps tend to work.

If you don’t want to install a second unit, you can try electric baseboard heat and a through-the-wall air conditioning unit. The air conditioning will be noisy. You might want to call either your utility company or the service technician for your current heat/ac and ask for advice.