I've used one of those smaller Ditch Witch trenchers, and they'll get the job done. But it will be a workout to trench 250 feet of wet clay, especially if you have any tree roots to deal with. (I'm assuming that in LA, wet is the only variety of clay you have.) I see that the nice man in the picture happens to be trenching in dry sand; you may not have the same expression on your face when you're behind that trencher. Make sure they show you how to engage the differential lock. Also, I happened to notice that our local HD had the chain backwards on one of its trenchers, which I'm sure made for great fun for the next guy to rent it!