I'd buy a trencher in a heartbeat. A decent, usable trencher would have to dig down at least a foot. To get to that depth, the wheel would have to be probably 30 inches in diameter to allow room for the drive around the axle. That's big! Probably need rear hydraulics to drive it (PTO drive would be difficult to set up, I think).
What's so wrong with the Ditch Witch-style approach? The patents have expired so it's not proprietary any more.
regards,
Mahlon