as little as you are going to be using it, get a tow behind from a rental company and bang them all out in day. some guys will try to use a regular air compressor because they figure it gets enough psi, but you need more cfm (cubic feet per minute) than these can give. A towbehind unit will blow out a typical residential unit atleast 3 times faster than a regular compressor and do a much more thorough job at it.
If I was you I would just sub it all to an irrigation contractor, you can make some money on the mark-up and be sure it is done right. Last thing you need is to have to replace a bunch of backflow preventors next spring. Most states require a plumber to replace backflows and pull a permit to do so, and thats not cheap
Why do people not respect us as they do other tradesmen? Because every Tom, Dick, and Harry doesn't think he can be a plumber or electrician!