The actual time needed to wipe out Windows and install is under 30 minutes.
See above.
Prebuilt machines use OEM keys, which are $10-$20 at best - whoever you buy the system from is definitely getting a volume discount. In my opinion, a small sacrifice to be free of M$.
Pretty much all modern Linux installers offer to wipe the existing drive and overwrite. Just prepare a USB drive and boot from it, then follow the steps in the wizard.
I GOT THE POO ON ME