150$ is rather ambitious for what you are describing as a custom made low power server. Managing to build something… Anything commercial out of new, hell even refurbished parts that has enough horse power to run anything more than a pihole/DNS server at this price point would be a challenge and a half. If you’re going refurbished/2nd hand, you’re likely gonna spend half of that on just shipping the parts to you.
I believe you are vastly underestimating the price of new low end parts and vastly overestimating the capabilities and availability of old micro servers. I’d say something like this would work at a price range of around 300~400$ (and even that’s ambitious imo).
And even then, that’s a NICHE audience you’d be targeting. It would be people who don’t wanna pay subscriptions, but also don’t wanna be bothered to spend a day or 2 figuring out how to set up a simple linux box on an old computer they have. I’m not saying that audience doesn’t exist, it’s just veeeeery niche.
Because linux doesn’t have an “ecosystem”. You have to either make your existing ecosystem work with linux or center your ecosystem around apps and things that work with linux. I do that and I’d say it’s more standardized than both the windows and the mac ones…