If you rent a trencher, get a self propelled one.

I didn't know how heavy they are and it was a real killer because they are so heavy. Hard to push and pull on level land and next to impossible on slopes.

Also, get one wide enough for the job. I was laying sprinkler lines and figured that 3.5 inches was enough. It wasn't and I had to widen by hand.

I had to dig several hundred feet of trenches and wish I had rented one on a Bobcat or something self propelled at least.

The rental shop or Home Depot etc. can tell you how it will work in rocky soil depending on how rocky. My yard has rocks but none are huge.

I have used a trencher twice. First time I used a 4ft. manual steer. The trench was 60 ft. of soil, 18 inches deep. The project took 45 minutes. Steering the trencher was cumbersome. Otherwise well worth the money.
My second job was much different. I knew I had a lot of rocks so I rented the 8 ft. trencher with power steering. The job was a nightmare. It took 12 hours to cut 50 feet, 18 inches deep. The rocks I delt with were at most 18 inches in diamater. The trencher could not lift rocks deeper than 10 inches. The ones that got lifted would cause the machine to shut down. I spent most of my time picking by hand.
If I did it again I would use a backhoe and put down sheets of plywood to for ease of cleanup.

I rented trencher a few years ago and dug some trenches for sewage and water lines.probably 300 feet or so. All completed in (1) day. It was easy and not too complicated but not too much rock,(Don't know how big or how much rock you have) mostly hard mississippi clay.The machine was a "ditch witch" and was self-propelled and a strong machine. Worked great, just be careful trenching on hills and stay away from the diection the trencher is moving, cause the trencher keeps coming even if you fall and get in front, if you know what I mean.Best to have someone with you to shut the machine down in case you fall or something else happens and you are unable to get to the controls. This was the only time I have used a trencher and learned to trench up the hills and not down the hill for safety reasons.They can be dangerous, so use caution. Be careful and good digging.

Remember to ask your local cable/electric/water company to come out and mark your property. You can do a lot of damage with one of those.

have a look on this
www.erkat.de/en

Here is a link that might be useful: ERKAT

I can't address the rock issue but for a sprinkler system it's the only way to go. Never had much problem tearing through good sized roots.