Bigcut, there are a lot of variables only you know. For rental cost contact whoever you'd rent from and ask. Probably a 3 hr min. cost varies by location.
A trencher can did as deep as the digging bar allows, but I'd suggest you bury an electrical line in conduit and 24-30" deep. Then you have no worries in the future of hitting it. The conduit is an additional cost at installation, but you never have to dig up the line to fix something. So that saves time, cost and grief in the future. At the ends of your conduit use a "sweep". That's a long curved 90 deg. bent (thus called a sweep).
Always dig a couple of inches deeper than you want the line then you just leave the crumbs in the ditch and lay your conduit/line. If you have a code you have to follow and it states a particular depth of bury, that measurement is from ground level (lay a shove across the ditch and measure down to dirt) to TOP of pipe, not bottom of ditch.
When a trencher hits rock, once again, lots of variables. How big is the rock, how many of what size? Rocks the size of a softball, the trencher will remove pretty easily (although it will bounce it a few times until it clears). Large rocks that are wider than the trench you need to raise the boom and take a rock bar to crack it and remove with your hands.
I'm describing a ride on trencher, not a bicycle handle bar'd dirt scratcher! A trencher (the size I'm describing) is easy to operate, has a "creep" hyd. motor for movement and digging. You can line the machine up from your starting point to your target area (if it's a straight line) and start digging a foot away from where you want to actually have the line come up into the panel and then use the creep it inch back as slow as you choose to get closer. You can get off the machine and use a sharp shooter (spade) to knock the crumbs off by the pole or conc. wall to eliminate all other digging. ALWAYS have the digging chain out of gear (not turning) when doing any hand work.
A ride on trencher will have a backfill blade. Always a 4-way, sometimes a 6-way. A 30HP trencher has a backfill blade with a small lip curled to the front. This allows the operator to put the blade all the way down to original level, angle and push the material back into the ditch and not dig into the undisturbed ground. With the machine I'm describing to you, you can backfill a 150' trench in a few minutes. Like up and back and use the weight of the machine to go over with the tires to make it look almost undisturbed except of about 5". Total time backfilling is minimal.
Oh, when you're digging, at about the middle of your run (or where ever it's convenient) dig a few inches deeper for about 10'. Then pull up and go to the other end (where you were going to finish) and do just as you started before. Then move toward the cut you already made. Just line up and go right over where you dug the extra depth. Continue digging in the previous dug trench for 8-9'. Now pull the digging boom up, pull forward a foot or so, and back over the ditch you just dug. Get all the way across the ditch and out of the way.
If the trencher has a backhoe you can use that to clean out the loose crumbs, if not use a spade to get it out. This is the most work you'll do digging a trench with a trencher!
You'll increase your competence and confidence in operating any machine and have a nice, long lasting installation!!