hp microserver replacement advice

[encephalopathy]

i currently have a n54 microserver acting as a download box running windows. it really struggles especially if i have to unpack stuff. It originally was my storage box but i have moved onto a dedicated NAS (ugreen 4800). It is time to retire it!

What should i go for second hand, processor/mobo and ram-wise to comfortably process moderate workloads? i5,? i7? etc
Ideally need space for at least 3x3.5" hdd

thanks