In scenarios such as these what would be the recommended backup approach: DAS, NAS, Backup Appliance, lower end server, removable disk storage, tapes (intentionally left out cloud)?
Should be separate (physically!) entity non-related to your production cluster. Cheap NAS is OK.
For the average scenario (and I really just mean average) it's Synology or ReadyNAS that I recommend. Easy, supported, cost effective, desktop or rackmount options, well known, good brands, nice features.