rowland, welcome to being an owner. Many challenges await you, both good and bad.
Let me start of with this: http://www.veteranscorp.org/
Go to this site and read some of the info there. I know it is a veterans site, but there is plenty of free info there for anyone.
I reccomend reading it a couple times.
Service contracts. Sound good to me. Alot of AC guys do it, and make money on it, so why not you...There is a market for anything these days, depending on how much effort you put into researching the idea would be how much gain you get out of it..