I have a different view on this subject. I do lots and lots of heavy equipment. I would say go with a piece rate. As you get better and get them on a regular schedule your hourly will go up. You could never sell $250 an hour but with per piece I can do that easily.
Also, scraping large chunks of grease off first will help to keep the mess as minimal as possible. Watch out for stickers, service histories, windows, and hand rails. we wash the windows and wipe down the hand rails every time. no operator wants to climb in and not see or get crap on his hands. You also have to decide for yourself how clean your going to get it. Remember its heavy equpiment and going to look like crap in a few days anyway. You still need to do good work, but it doesn't need to be like a vehilce. Tons of up sells, the insides, engine clean, reclaim cause not many will do it and one day you'll get hit.
pm me if you want some more info...