Hi, I'll be using this setup to stream my Plex movies, 98% of which are 1080p (so no 4K). It'll only be me who is streaming, no 3rd parties. I don't really care if it's an old model as long as it works fine. Can you advise something on this information?
Based on the information you've provided, all three models you mentioned, the DS220+, DS420+, and DS423+ should be able to handle streaming 1080p videos on Plex with no issues, since they all have capable CPUs and sufficient RAM.
However, if you don't care about having the latest and greatest model, the DS220+ and DS420+ should be perfectly fine for your needs. These models are well-proven and have been on the market for a while, with many users successfully using them for media streaming.
Both the DS220+ and DS420+ come with a 2-core Intel Celeron J4025 processor, which is powerful enough to handle your streaming needs, and they have 2GB and 4GB of RAM, respectively. The DS220+ has two drive bays, while the DS420+ has four drive bays, which may be a consideration depending on how much storage capacity you need.
Overall, if you don't need the extra performance or features of the DS423+ and are looking for a reliable, proven option, the DS220+ or DS420+ should be a good fit for your needs.