Building a computer from the ground up only takes an hour or so but loading the operating system, then all the Microsoft updates for it, followed by all the usual useful software (like Adobe Reader, etc) and configuring the Internet connection, loading and updating an anti-virus programme, firewall protection, etc. usually takes another 3 - 4 hours.
Windows 11 is now Microsoft's latest operating system and appears to have a nicer interface than Windows 10. Of course it's early days yet as to how it will be received and what it delivers better than Windows 10 currently does, other than it's more sophisticated inbuilt security, but Windows 10 is unlikely to be supported by Microsft beyond the next 5 or 6 years anyway so Windows 11 is the only choice going forward.
If you elect to install it through your Windows Update on your current Windows 10 machine do a little homework online first as some software and/or peripherals you use now may not be compatible with Windows 11.
The primary consideration when selecting components is to ensure as much "future proofing" (upgradability) as possible for a given budget. This also depends on what you want to use the computer for. If it's just Internet surfing, email and some word processing (ie: letters, business documents etc.) then you don't need a superduper machine but it still needs to be fast enough to cope with present and future programmes.
If you are into "gaming", offline or online, then you do need a machine with "grunt" - particularly in the CPU (Central Processing Unit), RAM and Graphics areas. These don't come cheap of course but can be immensely satisfying - especially when the computer "waits" for you rather than the other way around.
If you are considering having a purpose built computer talk to me first.