I bought a used HP MDS 600 on EBay for less $2000 shipped.
It doesn't have a computer built in, but unlike the Backblaze it has four power supplies, two redundant and four fans and two SAS interfaces. It also holds 70 drives. I've thought of building a Backblaze before, but if you want to meet your storage needs with a single enclosure, there are better solutions for the money. The Storage Pod is really for when you have enough of them to consider any single one redundant.
Obviously a one-off is always going to be source-able from second hand bits and pieces. But if you need to build a these on a regular basis and you need to maintain them then uniformity and slightly higher cost are not an issue.