Water pressure and drainage will be your biggest enemy, im just learning as i go along at the minute but ive done one job where the tap flow was terrible and i must have wast an hour waiting for filling...another where the drainage was awfull sloping drive to the garage and no where for it to go. Both things slow you down, I used a gutter vac and a brush to get rid of the water but ive seen people use dirty water pumps and a gutter sucker thing that attaches to your lance.
30lpm is only going to win provided the tap pressure can keep up but you have the extra umph there if you need it so if you can afford a bigger machine why not, it seems like you are hell bent on getting one so just do it.