Technic motorised

Hi I've just ordered the 42177 g wagon and waiting for it to be delivered! Has anyone motorised one of these? If so, 1) where did you buy the motor set? I can't find anything clear- only AliExpress etc are coming up for me. 2) Did you fully finish building the set first before motorising or is it something you should be during the build? I don't know much about the motorising but it's something I'd like to get in to and re do a few of the cars I've already built submitted by ...