What hardware should I use ?

You don't need phenomenal power to self-host. Start with a machine recovered because it is too weak for office use.

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.
Tips aboutAPU2