Hello, I'm looking to have Plex Server for myself. Please recommend me the best Plex Server (not using the PC) but something like RasberryPi or something which is compact in size and uses very low electricity.
I don't think this particular model (215j) is available now but FYI, it cost me $300 about 3 years ago (got it from USA through a relative). But a 2 bay NAS like mine should cost in the range of 16000-18000 Rs. from Amazon or suchlike. I purchased a 3TB Red WD to go with it which costs around Rs. 8000 these days but iirc cost around 7000 at that time. Hard disk capacity and brand is, in any case, your individual choice. I shucked an old 2TB seagate from an old Blackarmor nas I had and added it to the 2nd bay. Regarding your second question, Synology NASs have a variety of packages available. I'm currently using Plex and Drive. But in the past I've used Download Station and a couple other. You'll find a full list of syno packages here: Packages | Synology Inc..
Synology has far more features than WD because of its OS. When I first bought it, I barely used any of its advanced features but my use has evolved over time. The different models you have listed differ mostly because of RAM. Depending on your budget, you can go for 218Play or if you are short on the budget, the 218J. Of course, if budget is not a constraint, 218 or 218+.
I think it would also depend on what you expect from it. If you need it to transcode video, a basic NAS is not going to cut it. But If you just need it to stream videos as is (provided your client/player supports it), then any basic NAS device should work.
For transcoding, you'd need a device with good CPU. A sufficiently powered NUC as @Anurag Bhatia suggested world work. If you want cheaper options, you can look at Mini PCs on Aliexpress. I use one myself, it's a low end Celeron N3150 model. I have Plex installed and it works great in most cases, but the moment it needs video transcoding, it starts to slow down.
Probably a newer gen i3 or i5 based Mini PC would work great.