Dell has refurbished Optiplex's 50% off right now.

This one is $164.50 with the coupon code HOTDEAL3070

Or here are some other deals on 3070s.

There are even some 3050s for as low as $55 with the coupon code HOTDEAL3050

A Raspberry Pi will not be good enough for streaming and the wireless adapter on it is pretty terrible. I tried using a Raspberry Pi and it was literally unusable for me so I bought a cheap Optiplex. I've been using a 3060 and it's been great so far.