Could be something weird going on with your cell provider and account there, could be someone has access to your iCloud. Check your iCloud and make sure you revoke access to any unauthorized devices and change your password to something secure and unique. If you’re saving your passwords to a Google account or other password manager, change passwords there as well and change passwords to any email accounts connected to that. Could also be a glitch, but the average person takes about ten seconds of Google searching to “hack” because of bad security practices so I’d take the chance to update that stuff.

$50 to maaaaybe $200 if you’re really lucky. More likely on the low end than the high end. Macs from that era and even a lot of newer ones pre-M1 have nosedived in price because Apple isn’t supporting x86 going forward.