Hi paul.
I pretty much do exactly as you do.
I have about 2 days per week of window cleaning work (150 or so custies)
i also valet cars, which is what i did originally.
I also clean carpets, but only "on the side" so to speak maybe 2-3 jobs per month.
I bought a second hand machine for £500, extracta exel...it does the job fine.
For a vaccumm i use my henry and for agitation i use a pile brush
the rest is knowledge, i went on the cleansmart course.
whilst i cant say im the quickest carpet cleaner or have the most efficent equipment i do manage to do the job perfectly well.
At the moment im looking to add a bit more window cleaning to replace the car valeting then next year push on with the carpets.
Yes there is decent money to be earned on the windows but i dont think i would fancy 5 days a week flat out on the glass, it get quite hard work the theres rain delays etc, messers, bad payers the usual.
I wouldnt walk into a custies home with a george (cringe) i have a george and use it for cars and whilst in theory you could use it, it would take forever and imaging the custies face.
So you dont nee to spend LOADS to get started but second hand maybe a grand all in would be a better start
one thing i would add is that of marketting, with carpets its pretty much going to be ongoing, with windows once your full your full and you can pretty much stay that way by doing very little, food for thought