Hi, I am a traditional window cleaner and want to try a wfp system, but wanted to try a backpack first, because of expense and just to try the system. Would a 18lt backpack be a good buy. It says it has a pump capable of pumping up to 60ft high. How many houses do you think I could clean with one tank full? It is £125.00 new, do you think this is a fair price?