Hi nfamus10. Since you're still in the process of building your garage-condo, how often do you plan to use the place? I think the way you heat/cool your garage-condo depends on your usage and preferences, too.
I'd prefer a new unit, of course. But, before installing a new unit, try to live in the house first. Let your unit work for a few days and monitor its performance. That way, you'll be able to determine if it's still working well. :)
You'll contact an HVAC guy who'll do the thing on the other unit, right? I think you should also let him inspect the unit, too, just to be certain that the unit has no issues.