Mike
At first I would look at what type of houses you will clean before you decide what type of trolley. For example if you were to do the old back to back houses then the major problems that you will come across is how to clean the backs, you will find that the back alleys are full of rubbish and can be quite difficult to gain access, for this type of property you would need a backpack or run a hose though the house.
The next problem is most trolleys are limited on the amount of water they can carry normally it is 25L or 50L and if you got to clean a 3 Bedroom house it will take at least 25L for a first clean and about 18L for every other clean, if you are cleaning a newly built house then the amount of water used can increase upto 100L, so a van mounted system might be useful here.
The next problem is parking you will find that somethimes you can't park outside your customers house and the nearest parking space could be up to a quarter of a mile away or more.
So at first I would look at what type of properties I will clean, then look at what system would be the best for it. Also don't forget the Kerb, some trolleys on the market have small wheels and you can't get them up or down some kerbs.
William
By the way I have developed a complete trolley system which should be able to cope on whatever cleaning job it is given.
Its going though its road trails at the moment but it will be on the market within the next 6 weeks with a price tag of £1000 including everything you need to clean windows from making your purewater down to the poles to clean the windows.