Hi there
Simple if you have a DECENT smart phone with an acceptable size screen which allows HDMI output (a lot of HDMI adapter cables are available for most phones -- cheap too around 8 USD).

1) Video / Music content -- transfer or copy to phone's micro SD card --these are available in sizes up to 32 GB at the moment -- soon they will be available in 64 and 128 GB sizes too.

2) For output to a large monitor / TV / Screen -- get a HDMI adapter for your phone --the output is often better than 1080p full HD.

3) For keyboard get a folding blue tooth keybard and pair with the phone.

I'm replying to this forum using a phone in this way. It plays movies pretty well too from the Wifi or even from local storage on the micro SD card.

To replicate a mouse is a bit tricky but use the phone's touch screen.

My PC usage these days is around 70% less than it was a year ago. !! Basically I only use a PC at work these days or if I want to get files from my HDD's to the phone. In the future I'll probably use NAS only so even here I won't need a PC.