Keith,
I did duct cleaning for 10 years, mainly as an owner / op. Duct cleaning usually picked up when carpet cleaning fell off. It was a great source of income when I started since there were only 4 companies in Indianapolis (12th largest city in the US) that were in the yellow pages so I got alot of calls. I took the first duct cleaning class that Unsmoke taught. That was in 1993, and none of the HVAC companies did it. Now its a different story, all of the larger ones do it, along with cheapass carpet cleaners. The bulk of my business came from HVAC and fire / water contractors. My minimum was $300.00 and that was for a couple hour job (easy). My biggest was a $37,000 job in a retirement home that took about 3 weeks.
I started with a portable electric hepavac and compressor in a trailer which was inexpensive but difficult in some driveways. I then bought a repossed truckmount that sold for $65,000 new. I got it for $22,000 with 9000 miles on a NPR Izusu diesel chasis. It was made by Altanic Engineering which makes portables and slide ins for ducts. I kept that truck for 8 years and sold it for $36,000 along with the biz name of "Airemasters".
Dont even think about using an accessory for your cc truck, they are a rippoff as is a rotojunk. You can always lease stuff cheap, or find used. The best thing you can do is get hooked up with the best company in a non-competitive market and do a work with. Your distributor may be able to help.
You need to test some waters as to where your business will come from before jumping in. I got out for a couple of reasons. My biggest customers were getting into it, and its a nasty dirty hard job (hope you like crawl spaces, attics, rodents, and breathing crap).
I can tell you anything you want to know about it so feel free to ask away or we can always talk on the phone.