If you are going to bury the pipe less than 2' deep, I would think a chain trencher would be best .... ride on is easier to use that a walk along type.
If I were going deeper than that, I would consider a "mini track hoe" with either a 9" or 12" bucket. That would allow you do dig up to 7 feet deep.
At that depth, you may not get much, if any, pipe movement due to temperature changes.
I also install irrigation for a living. When we did Condos, many of then eventually end up with several thousand feet of mainline, we rarely needed larger than 2". Since we were digging the mainline with a hoe, we went deep, because we could. The little extra pipe you need for coming up is negligible.
Since Florida does not really have a freeze issue, we did not have to worry about that. If it was a concern, I would probably use a "slip fix" where the pipe leaves the ground vertically, this would allow movement without leaking.