Plex is a great service for playing files stored on your network to a variety of devices. They have apps for pretty much everything at this point, including Roku and the new Apple TV. However, the only Synology device that appears capable of transcoding 1080p files seems to be the DS415+, which has an Atom CPU. Even then, it seems to struggle and max out the processing power. But if the file doesn't need to be transcoded on the fly I don't think it's as much of an issue. Tony had a thread about this a while back.

I personally like Crashplan a lot. I've used it for about 6 years now, and in that time I've only done about 3 or 4 file restores, but one of those happened to be every photo and video I'd taken for the previous ten years, so as far as I'm concerned it's worth every penny. It's essential insurance.

I'm not sure if an internet connection is required for backup over the LAN. It's certainly not necessary for backing up to local disks.

IMO, a NAS isn't that essential for backups. I'm fine with just a cloud backup. The problem I have is that I store all my data on my NAS and almost nothing on my actual computer. The problem with that is that Crashplan isn't officially supported by Synology, and the unofficial package I have installed on my system is a real resource hog when it's at its worst. It even crashed my diskstation the other day and I had to manually shut down because I couldn't even log into the GUI.