See What is Plex?
Plex is a client-server system. To stream media to your phone, tv, etc, you must have Plex Media Server running somewhere - on a computer such as a desktop or laptop, on a NAS, server, etc…
Transcoding is independent of where Plex Media Server runs. PC, NAS, etc.
Transcoding is required when the Plex client does not support the format of the media you are trying to stream.
For example, If you stream a movie with 1080p video to a device with a 720p screen, the video will be transcoded to fit the device screen size. Similarly, if the video is encoded at 20 Mbps, and you’re streaming to a phone with a connection of 5 Mbps, the video will be transcoded to fit the bandwidth limitation. Audio may also be transcoded. For example, a movie may have dts audio, but the client only supports Dolby Digital. Plex Media Server will transcode the audio.
Your best approach is to prepare your movies, etc so transcoding is not required. Doing so will put the least load on your systems and reduce chances of buffering and other issues. This may require using tools such as Handbrake to prepare the media in advance of loading it into Plex.
Be aware that if your media is stored on your NAS and it is turned off, you cannot steam media to your phone, tv, etc. An installation of PMS must be running and have access to your media to successfully stream to your devices.
Most people leave their NAS running full time. The ones you mention use very little power. They are designed to run 7x24 (use NAS rated drives as well).
Between those three, choose the DS218+. It has a Intel Celeron CPU, which supports hardware accelerated transcoding if you run PMS on the 218+ (Plex Pass required for hardware accelerated transcoding).
Review the NAS Compatibility List, and linked spreadsheet, before making your final decision. Note the limitations between the various devices and between software and hardware transcoding on each.
You can successfully run PMS on a NAS such as the DS218+ (I use a DS918+). Just make sure you are aware of the limitations of the platform.