I think with your goal being walking as much as you want, exercising exactly that makes the most sense.
Yeah. And you'll need a car to transport the thing, which will be driven by Kornelis or one of your parents in law anyway, so you can simply stay in the car and avoid people while they do the exchange of money/treadmill.
I strongly second looking second hand. If it turns out it isn't to your liking it's much better to have thrown away 60 euros than at least 300.
I agree with you on the one
@Zhnyaka linked looking good though. (And I absolutely love the concept of someone in Russia helping someone out on a UK website by searching in Dutch webshops!)
I had a look on Marktplaats for you, there's lots on offer under 100 euros, and like
@AndBreathe said, you can look up make and model before reacting. Those things usually get resold after having been used 6 times in early januari, they're as good as new and people want to get rid of them because seeing it every day confronts them with failed new year's resolutions every day.