Worth doing?

[LivingAlmostLarge]

We can't just put in ducts because it's a townhouse with shared walls.

[woodie96]

With the energy that you are using with the space heaters to provide a "comfortable" environment, I think that you are probably using way much more than the new unit will use to produce a "more comfortable" living environment.
I would say that if you can swing it, go for it. In a couple of years, the energy credits and possibly the rebates may go by the wayside.
I would look at it this way if I could swing the additional 2700 to do it. Least about half of it was paid for by other means.