Loved to build VERY stable Extreme OC PC's Liquid cooled and dead silent.
...this thing most be DEAD SILENT!
For $800 and an 8 year life span? What a wonderful dream! Even with all the parts you bring to the table.
Parts that will be provided or have. All OS's
Oh? Legally? The vast majority of existing Windows licenses are OEM - that is, they came with, or were purchased for use with a previous computer. The vast majority of Windows licenses are not the considerably more expensive full "Retail" licenses. OEM licenses are inextricably tied to the computers they came with or were purchased for and are NOT, under ANY circumstances, legally transferable to a new computer (or upgraded motherboard). So a big chunk of your budget will be eaten up by Windows licenses.
RAM and motherboards that support "VERY stable" "extreme" overclocking are not budget models!
You emphasize DEAD SILENT. That is not going to happen! And in particular, not with extreme overclocking. You can water cool the CPU, but the RAM, chipset, drives and other internal devices need cooling too so you will need sufficient case fans to create a nice flow of cool air through the case to provide sufficient cooling to them. And most PSUs have fans, as do most graphics cards too. Even the radiators for most water cooling systems have fans.
There are "DEAD SILENT" PCs. These are typically HTPCs (home theater PCs) that use "passively cooled" (no fans) PSUs, CPU coolers and graphics cards and special cases designed to take full advantage if heat's natural characteristic of rising. And I note many audiophiles who build HTPCs that use fans to cool the CPUs underclock the CPU to keep it cool enough so the fan will stay off or at most, spin very slowly to avoid any fan noise.
No mention of keyboard or mouse. Most controllers use USB so you likely will not need a game port. And virtually every motherboard comes with decent sound and the better boards (the ones that support "extreme" overclocking) come with excellent integrated sound. So you don't need a sound card now either.
As far as your 8+ year life expectancy requirement, that typically is not a problem with hardware - if not abused! That is, if fed good, clean stable power, if not pushed with extreme overclocking, and if kept cool (see the last line in my sig). The problem with your 8+ year demand is the rest of the world will not sit still during that time. Most electronics becomes obsolete and is "retired" because it does not meet or support the latest games, operating systems and other programs, latest I/O protocols, latest state-of-the-art and other current demands - not because it failed.