If you're going to be doing mostly two story houses, by far the most versatile ladder is a 3.5m double, preferably a 'tree bottomed point' - the traditional window cleaners ladder.
This will reach all but the most awkward first floor windows and because you can close it down it is easier to carry. You can also split it and use just one half on those slightly higher bungalows.
Other than that I would advise a 4.5 or 5.0m double which will be long enough for any other jobs, but you will only need to get it off the van a couple of times a week.
Also consider fitting (or getting fitted) some swivel rubber safety feet. Midland Ladders do some excellen ones which are oval shaped and have big numptyby bits on the bottom :
Put a pair of these on a 3.5m double splay footed point and you've got the best possible bit of kit for the job
