Accept my condolences too.
Once you get to a state where it has power you can either clone the disk or remove & replace the HD/ SSD as others have stated. Target Disk mode is helpful if you have another Mac & the required cables.
How to use and troubleshoot FireWire target disk mode - Apple Support
As to the data & user accounts here is how OS X handles it by default…
The user account password can be reset in recovery mode. Then you can login as that user.
http://osxdaily.com/2011/08/24/reset-mac-os-x-10-7-lion-password/
The keychain saves the users passwords using this default account password but it is not reset when the account password is changed, you need to find the original password to access the passwords. You can however read the locations & usernames of keychain items, so it could help with account resets assuming you can control the associated email accounts. Keychain Access shows the keychain items.
It's possible that FileVault is enabled in which case you cannot decrypt the disk without the password or the master file vault password. I think this password could have been saved in iCloud, the setup offers for Apple to back up this critical piece of info. You would need access to the Apple ID.
File Vault encryption also prevents the account passwords being reset.
There may also be a firmware password, this will prevent any startup key combinations, so you cannot boot from other disks etc.
It really depends on how conservative the user was, good luck with it & let us know if you get anywhere.