1. $17-25 seems reasonable. I believe most exchange stations charge $18-20 around here, and that's you exchanging it yourself.
As for whether or not I would be interested; probably not. My situation isn't really relavent to the survey, though. My stepfather's equipment rental company, among multiple other odd things, fills propane tanks. 20lb. all the way up to 100lb. cylinders. We charge our customers $0.65/lb. ($13/20lb.) We get a TON of business, and it really helps during the slower winter months to supplement the lack of equipment renting we get. We pay roughly $0.40/lb for it, and even with a 62.5% markup on it, we're still much much cheaper than other local fill stations. So, main reason I'd personally do it myself; I can do it much cheaper.
If I wasn't in the situation I'm in, then sure, I might opt for a service such as yours.
2. I've found that most of our customers either: a. see our sign out front of our building that says we're a propane filling station, b. they see an ad/coupon in the local newspaper that we might run once every few months, or c. they hear about us from friends and family.
Radio wouldn't hurt either if you were in the business of strictly propane delivery, IMO.
3. Not really sure here. Purhaps selling/renting construction torpedo style propane heaters as well as selling propane and renting/selling tanks?
4. Can't seem to think of any problems with the business. Purhaps licensing, regulations, and any fees you'd have to pay to the gov't to be allowed to transport large quantities of flammable gas.
Also can't think of any reasons why it wouldn't spread. Perhaps the economy is causing people to pinch pennies wherever they can?
Final thoughts: Make your delivery trucks run on propane. a. to promote your product, and b. I'm sure you can get it cheaper than gasoline.