I'm interested in starting out like this, I have a bunch of 2.5" SSD's I'm not using- do you have any tips on what cube to get? Are you concerned about power usage at all especially if this is always on?

Any of those cube gaming ones I think are great. I got a dual chamber one which makes shuffling drives and cabling easy. Can't remember the name but it was 90 pounds, way more than I paid for the old 4U, although inflation from the 90s probably means it was more expensive in real terms. Most of the power is used to spin rust so not sure it's worth worrying about the HW power use, just use whatever old pc you can get for free, ask colleagues and family, people throw out working PCs all the time, it's a NAS, not a rendering farm, if it boots it's good enough IMHO.