Need help to determine best heating system brands and price or possibility to connect to old system.

If your duct system has dampers in the branch runs you can redirect the air flow, but do not close them off completely. If they start to whistle they are closed to much. All systems have to be manufactured to strict ARI standards. You probably don't need all the extra bells and whistles on a unit. Depending on the construction and insulation in the addition, I would reccommend you look at installing a mini-split heat pump system in the 600 sq ft area. I would not think about using the existing system because was not sized for any additional construction when it was installed. Let me know if you need anymore info. I hope this answers your question.

Thank you stanhvac1. This was very informative. Could you please let me know where I could learn more about mini-split heat pump system. Do you know how much they typically cost? 

There are different companies that manufacture mini split systems. Here are some links to some for you to look at and get some more information.

I have not installed a small unit that you would need so I can not advise a price for you. The ones I have installed are multi zone 2 to 3 ton units. You would only require a single zone 1 ton unit for your situation. I just installed a multi zone 2 ton unit in our office and the total price for all the equipment, which is equal to 3 systems was $3,500.00. You will need to get an estimate from a local contractor in your area. Hope this info  helps. If you need any more info or have any questions feel free to contact me. Here is a link to my website where you can get contact information if you need it. Good luck!

Thank you