leepaton1,
Tread carefully, Sad but true the customer is most important here. without them you wont be earning. Dont lose sight of the fact that what it all boils down to is cash in your pocket.
Is this work established? do you know how to wfp? what are the customers used to?
If its established wfp then go for it, and just use whatever system you can get your hands on, they are all as cheap or expensive as you want them to be. However if its "fresh" work or been done previously with a ladder I personally would establish myself with the customer as a good, reliable, honest window cleaner and use a ladder. (with a definate goal of switching to wfp within a year.)
Make sure you keep your 120 customers and make sure that number goes up not down.
Ps: A personal experience:
I bought 600 customers a few years ago, worth about 4k a month (established work, trad, and a little underpriced) but a nice round. It took me about 3 months to totally decimate it. changed to wfp, done a rubbish job and lost over 400 of them. Didnt even have the sense to tell them I would do it trad from now on, because I was so adamant I was wfp from now on!!! With a little forethought I could have kept those customers, raised the prices over a couple of years gone wfp and had an awesome round.