I'm not so sure about this really. I read your post that you only want to use the pure water method for the odd few windows (presumably doing the safer windows from a ladder?). If so, DI would be perfectly acceptable rather than messing around buying and setting up an RO unit. However, if you feel that you would want to increase your pure water usage, it would be better to get an RO once you are ready to make that happen.
Be warned that once you start seeing the benefits of WFP, you probably will want to use it on all window - or at the very least, all the upper ones.