For a great job that is competitively fast, a good degreaser and hot water is always great.
I have never had good results with just hot water or a steam machine without degreasers or other chemicals on heavy equipment that was going to be serviced or painted, the mechanics or painters are very picky about this stuff.
Give it a try and see how you like it, no matter what anyone says, it is you doing the job, making the money so it has to work for you, be efficient but mostly, it has to do a good job that you are putting your name onto.
Something to remember is that you might have decals, paint, hoses, wires, etc.... that you have to think about that might be there also and you really don't need 300 degrees to clean engine bays or heavy equipment. I have never heard of anyone out there that does this type of cleaning that says that they use water that hot for cleaning. There is no need for water that hot in probably 95% to about 99% of most cleaning jobs that you are degreasing engine bays, heavy equipment, vehicles, etc.....
There is a lot of mis-information out there and misconstrued information out there, look into the type of cleaning you are going to do and always consider the sources of your information to be sure they are apples to apples and not apples to morons.
At the Las Vegas event years ago, we used a machine that was making over 300 degrees hot water and guys were trying it out to remove gum. We were told that over 250 degrees the gum would "vaporize" and we would be amazed. That was very funny! hahahahaha many guys got to try to remove gum at the Vegas DMV and suddenly realized that hard, old, baked into the concrete gum is harder to clean than fresh gum or gum that is just months old.
When Tony Shelton started up his Landa hot water pressure washer, the gum came off at about the same speed as the machine that made water over 300 degrees, I tried both, Nigel tried, Tony tried, Chris Shelton tried and other people tried as well and there is even a youtube video of this happening, I might be able to find it and post a link later on but my point is this, you don't need super hot water to do degreasing when you use some good degreasers.
Good luck.