Not enough details here to really help, but even if you did give more details it would be hard to give you specific advice. You need to have contractors out to look at the system and give you quotes for replacement. I'm assuming this is a standard gas forced air furnace and coil in the crawlspace matched with standard A/C condensing unit outside?

If so, you could just get a new unit in theory, remove the old one, plug and play, but there may be caveats and things that need to be changed. Your duct system may "work" but need repairs or replacement to achieve optimum performance and efficiency. Only a contractor inspecting the system in person will be able to determine this, although different contractors may have different opinions/strategies.

Personally, I'm not a fan of duct cleaning. Generally speaking, if dirt is stuck in the duct, that's where it will stay. Duct cleaning may make the situation worse by agitating it. Your ducts may need repair/replacement, but that would be for reasons other than dirt.

This post was edited by hvtech42 on Fri, Nov 14, 14 at 19:19

I am also not a fan of duct cleaning, but there is going to be 19 years of dust sitting inside those ducts.

How has this house been heated since the furnace stopped working?