I swapped out my 2 HTPCs for Raspberri Pi 3 with a 64Gb SD card and OSMC/Kodi...linked up to my QNAP NAS. Never looked back. I use an HDHomeRun for TV (used to use WMC), Plex for digital media, and MCBuddy to strip out the commercials of my recorded TV.
The hardware headaches are gone, the bluescreens are gone, etc.
Rather than start with a budget and a new laptop as the answer, start with what you need to do? Watch TV, watch 4k content (which I know you indicated, but is that a reality), stream media, surf the web, etc.
Once you have built the list of what it MUST do and what would be NICE to do...then you can start looking at solutions.
my 2 cents...