Youll need a monitor and keyboard initially but can go fully headless after configured.

Refurbished Dell Optiplex SFF 5050 or similar model from Amazon is power stable and much faster than a raspberry pi.

Add a 2TB or larger NVME. Total cost about $175-$200

Setup Ubuntu 20.04 or newer.

For software, install Nextcloud for your own cloud. Remote access with phone apps is doable if you setup the certs as long as you can get a static ip.