Is the insulation around the duct work in good shape? 

I want to keep the duct-work in case I need it later. 

They have to have a water source , cut into the ductwork .Its not a DIY project .

Is there already ductwork through-out?

Your talking new HVAC (ductwork?)

I can have an entire system put in, including ductwork, for $5k or so, maybe a little less depending on the setup.

I assume this price does not include replacing duct work.

This is what causes the motors to break - they are just over worked.Furthermore, the electricity that it takes to run the fan exceeds the energy that you save from cooling the attic - again, regional differences may occur, but this is at least true in the south - see this study done in Florida:http://www.fsec.ucf.edu/en/publications/html/FSEC-GP-171-00/You do get benefit of reducing strain on the HVAC system and ductwork if it is in the attic, so even though you are spending more on electricity, reducing the temperature in your attic is important because this will increase the longevity of your HVAC system (if it is in the attic).So, the conclusion is that for energy savings, solar powered attic fans are superior.

Forced hot air required a furnace and ductwork.  

Minor wall damage.Here's my spitball repair cost estimates:Plumbing backs up in shower 300Entry Doors (2 doors) 1000Replace windows (9 windows) 3000Replace water heater 700Replace HVAC/Install Ducts 5000Replace Cabinets 1500Replace Countertops 500Replace Stove 300Encapsulate crawlspace 2000Replace flooring 1400Paint/Drywall repair 2000Total 17700I'm pretty confident in most of the costs here, but if anyone has any additional feedback on the cost of new HVAC duct installation and the crawlspace encapsulation I'd appreciate it.Sidebar: I'm thinking it would be better to encapsulate the crawlspace and then run the HVAC duct work through that area rather than through the attic.