Forget servers, buy a used laptop instead. That way you have a built in UPS in the form of the battery for uninterrupted staking.
For staking HDD is fine, but with a bunch of wallets you might want an SSD.
Just a qucik example:
Refurbished HP Laptop EliteBook 2560P Intel Core i7 2nd Gen 2620M (2.70 GHz) 16 GB Memory 256 GB SSD Intel HD Graphics 3000 12.5" Windows 10 Pro 64-Bit - $381 on newegg (
link).
At least this is what I would do if I didn't have a UPS already.
I do have an older Dell laptop,
https://www.newegg.com/Product/Product.aspx?Item=N82E16834295366&cm_re=dell_e6410-_-34-295-366-_-Product something like this but without SSD. My problem with laptops are, their cooling generally is shit, I don't know how long they can go 24/7 with that cooling, especially when it all gets hot, cpu warms up hdd, and it all gets damaged faster than a normal PC, even the power adapter of a laptop is insanely hot, like an 1A phone charger charging at 1A full power, after an hour tops it's close to melting. Also UPS doesn't really matter because if the power goes out, my router goes off too so net dies anyway, it's ok for very short power outages I give you that.
Do you use a laptop 24/7 for this task and you say it can handle it?
I was thinking something like a tower server but they all have quadros which is useless to me. If it were a new machine, would build something with small form factor, micro atx, big cpu heatsink and massive 200mm cooler on top so it would be cool and quiet, but buying this from new parts is really expensive, a decent cpu cooler is above 50$, same for case.
Btw, I can't buy from USA, I'm in EU.