Replacement of AC ductwork (main run)

I bought a new heatpump in Jan of 2007 and with that I replaced the ?runoffs? (the ones that go to each room). I couldnt replace the main ductwork at that time for financial reasons as I paid cash for the Heatpump and runoffs. So, I would like to replace the main ductwork at some point but I am not sure of the technical name of it is. Seems silly, I know, but my mom lives with me and she has breathing problems and just looking inside the ductwork from the 2 return air vents tells me that the previous owners probably let there kids put all sorts of things in the ductwork, and there is a musty smell which I am assuming is coming from an opening of some sort from under the house in the ductwork. This is located in a crawlspace and has no inuslation around it (well its there, but moisture has caused it to hang down off of it) (and yes, I know that the moisture problem needs to be fixed, I am working on that, I am just slllooooowwww.)
How much could I expect to pay someone to replace this and is it something I could do with help or better left to the professionals?

I would guess that the main run is at most 38 feet long and there are 3 runoffs on one side and 5 runoffs on the other and there are also 2 return air vents one 12 feet in and on the side of the house with the 3 runoffs (but in the middle of the width) and one of the side with 5 runoffs )and also in the middle of the width)

Also, is there a certain gauge that is needed to do this as I had asked the guy who installed the heatpump to replace it but he said he couldnt get the same gauge material as what was there) <-- and that is a story in and off itself

Hello Shan:
As brilliantly as you have handled your other projects, I'm sure you can handle this one too.
First, though, you need to look into having the ducts professionally cleaned and re-insulated. Unless there are several huge leaks in the system. If it is metal duct, it could last a lifetime and save you a bundle at the same time.
Glenn

Is there a special insulation for this type of ductwork? and is the insulation something I could do myself? As far as leaks, all I would need to do is to make sure the fit is good and then reinforce with duct tape (and yes I know it isnt the sticky kind... lol) and then wrap insulation around the ducts? I think I have one picture of the ductwork under the house.

Not a very good pic, maybe I will get brave and take a new one......at some point

Hello Shan:
Duct insulation is 2" thick fiberglass with a reinforced foil face. It comes in a 4' X 100' roll. You can measure around the duct, add 4" for folding and staple it every 2". Yes, you can do it but it takes a special type of staple gun that might be cost prohibitive. If you use duct tape to cover leaks, remember it dries out in a year or so and turns loose. The 4" wide foil tape will perform a lot better but, if the hole is a crack over 1/4" wide, only a piece of metal screwed on will hold the air.
I can see from your picture that you really need the black poly film cover under your whole house. It would make movement under there much much easier and hold the ground moisture down to pervent the musty smell.
Glenn

Do places like Lowes or HOme depot sell the black poly film? And arent you supposed to take it up about 6 inches into the foundation wall? HOw do you adhere it to the foundation and what is the best way to 'seam' it together?

Oh, and the insulation... I assume you need to go to an hvac supplier to get that? I didnt see anything like that at Lowes they only had teh runoffs like I have.

Hello Shan:
Yes, Lowes and Home Depot both have the poly film. It should be run up on the foundation at least 6" and can be stuck and seamed with plastic roof cement (which is easier to handle in caulking tubes, you may need 6 of them).
Yes, the insulation would be at the HVAC supplier.
Glenn

Thanks Glenn!!!!!!