For $300 your options will be a bit limited. At that price for minimum configuration required I would check out the Western Digital My Cloud Series. The 4TB version below is just over $300.

https://www.amazon.com/Cloud-Ultra-Network-Attached-Storage/dp/B01AWH04EW/ref=sr_1_6

If you want to have a prebuilt NAS that can transcode media files, especially HD / 4K media files using Plex, then a more powerful solution is needed. The Western Digital My Cloud Pro series is a good option. The pro series is more expensive than the solution above (drives not included either) but it's also far more powerful, supports transcoding and allows for a great number of RAID configurations. If you don't need to transcode videos to stream to devices the regular My Cloud option above is great for pictures, documents, music, etc.

https://www.amazon.com/Diskless-Cloud-Pro-Network-Attached-Storage/dp/B01GLRX6C4/ref=sr_1_7

I used to be against cloud storage (it's my data and I want it all local! lol) but for items such as family pictures / video, and documents it's a great option. I started to grow tired of worrying about my NAS having an issue (using RAID 5 but things can still go wrong), replacing drives, it being stolen or lost in a fire, backing up files to BluRay discs, etc.

A yearly Office 365 subscription can be purchased for $60 and it includes 1TB of OneDrive cloud storage. The flexibility of being able to access all of your files from nearly any device is great. The subscription also comes with access to Microsoft Office so $60 a year it seems very reasonable.

https://www.bhphotovideo.com/c/product/1059696-REG/microsoft_qq2_00011_microsoft_office_365_personal.html