I've had no problem with getting the RAM (either new or just more eBay stuff) - and even got a replacement mainboard for relatively cheap when one burned up (it was easier to swap the mainboard than reconfigure everything for a new server).
The killer is the power consumption. It's better now compared to old enterprise gear of 20 years ago, but you can sometimes still pay for a brand new low-power system just in power savings alone.
Of course, if you have solar onsite that doesn't matter.
The power is part of why I prefer the embedded boards like the one I linked.
Especially putting one of these not in a rackmount chassis, you can opt for quiet cooling and fans that do not draw 10s of watts each. The idle loads can get fairly low, depending on your additional peripherals.
For very low power, I actually prefer small form factor machines, either used 1L SFF machines from the big OEMs (see STH's Project TinyMiniMicro[0]) or one of the many AliExpress mini PCs[1]. These sorts of machines tend to use low-power CPUs and a lot of the AliExpress ones are laptop parts.
As with all things, there are always tradeoffs. If you enjoy researching and bargain hunting for hardware, then this is a viable path. If you just want a machine that works, then this is onerous work.
The killer is the power consumption. It's better now compared to old enterprise gear of 20 years ago, but you can sometimes still pay for a brand new low-power system just in power savings alone.
Of course, if you have solar onsite that doesn't matter.