Personally I couldn’t see renting a power screed to do a 15 x 20 pad. Will he need the 16’ attachment to ride on the forms or will he be eye balling it as most of it was done in the videos. I doubt most rental places have that long of a base. If I was going to rent something I might rent a bull float and uses a long 2x6 to screed it. A first time DIYer would catch on quicker with a bull float and it would do the same thing the power unit would do as far as finish.