Seems expensive to me.
And you'd need to find out the TDS of your water locally to gauge how long that DI would last.
How long would it take to pay for itself?
My personal advice would be find a local shiner near you. Chuck that occasional job his way. He would (hopefully) reciprocate.
For carpets around here, I recommend a guy that did my house.
He's never given me a job.
I'm not bitter though.
It's win/win as you've kept your customer happy and they haven't googled 'window cleaner ....' that may just do carpets too.
I'd keep your money and look for a window cleaner on your locality to have a coffee with.