What hardware should I use ?
Start with a computer too old to be used to play games or office work instead of throwing it away.
If you want to buy new hardware, check first if it's suported by OpenBSD :https://www.openbsd.org/faq/faq1.html#Platforms
ARM architectures don't require much power.
If you don't know where to start, APU are quite amazing : not too expensive, small, silent, require less than 10W and well supported. Actually, this documentation is hosted on an apu2d0.APU2 board constructor.