Personally i think you'll need plenty of power for residential air duct/furnace cleaning (especially if the air vents are on the floor), since many newer houses are large, have many many air ducts but only have one system (which made up a good portion of our customers since the ones who can afford newer houses usually aren't too poor).
A slightly used custom built van/truck with a slide in TM should be a good start, but you'll need to have a wider search area and few months of finding. hypervac h2 is what i've seen mounted most in the newer trucks, i.e. merc van, ram promaster, and ford transit. It might not have the max power as a traditional pto to easily clean the leaky air ducts or furnaces with 35+ vents, but should be quite close (and often it's about the techniques and experience of the operator, not the equipments). If you are really on budget, get the heatseal's kubota CTM and mount it in a used cargo moving truck. It won't look as professional as h2 and a bit less ergonomic, but power wise quite similar. Either way make sure you find someone who knows furnace cleaning to find the suitable one for you and get it inspected before purchase.
As far as I know the standard price of cleaning air duct varies from $100 to 250, but it depends on the company and their professional skills. Don't you think that it's too much? By the way, I personally use air purifiers to clean air in my house because using them is much more profitable and convenient. Choosing the good air purifier is a hard task though. Good for me, I started to use https://dehumidifiercritic.com/best-budget-air-purifier/ in order to read critical reviews. So, I would recommend you to read carefully several reviews and then you will definitely know what to buy.
Air duct cleaning is a vital part of home maintenance. Our last job was a rental and when we arrived the furnace was so dirty it literally would not run. The Air filter was not secure and small return ducts under the floor to the blower housing made it easy for light debris to just get sucked into the furnace so disassembly was vital for occupant safety. The furnace was also full of paper and a piece of clothing like a robe belt had been sucked into the furnace fan and had wrapped around the shaft, seizing it. Burners kicked on and then cycled right off. Flue pipe had holes in it and was plugged completely with rust . After cleaning and sanitizing the furnace, Ducts, coils and disassembling the fan and cleaning we installed a crude filter holder so this would not happen again. They were actually lucky the furnace safety switches were operating properly as this unit was a cantidate for causing a fire. After we cleaned sanitized and re assembled and installed new flue piping the furnace lit and ran like a champ. We always cycle it off and make sure it works properly so on the second attempt it lit and then shut off. We cleaned the flame sensor with emory cloth and she then fired and ran perfectly. Furnace was about twenty years old and they will now get another twenty if they keep it up. I think many furnace guys would have sold them a new furnace because it did look pretty bad. But if you aren't cleaning and checking the furnace and air handlers for problems then duct cleaning isn't probably for you. I realize you want to make lots of money, we all do, but you do all professionals a disservice if you don't know what is going on. Dont feel comfortable disassembling a furnace as part of your duct cleaning? Then don't !!! If you decide to do it, manufacturers offer free training and make sure its quality training. I use a Caddy Vac and I will post some pics so you can see what we do. I would rather die than be a roto brush LOSER.
Check out RamAir.co to learn about our revolutionary duct cleaning and sanitizing system which does not require cutting into the ducts and gets you more repeats and referrals than any other system (It has the WOW factor). Our clients report up to $500/hour. Investment and training are very low compared to other systems. I can't post links or videos yet, let me know if I can help. -Charles