If you are looking for a low entry system to go into window cleaning I would recommend a backpack with a suitcase type trolley.
If you are targeting terrace houses, you will find that the fronts are great for cleaning, but access to the back can be a nightmare, normally you will find that they are either overgrown with plant life, or the householder store their junk in it.
The other problem is that on some terrace houses you go down the alley and have access into the back garden of the house you need to clean, but you will also have some where you go down the alleyway and acroos 3 or 5 houses to the house you want.
A backpack that can be detachable from the trolley is the perfect entry system into wfp, imo.