The OS installed is not necessarily tied to any AppleID. It might be. If you walk out of a shop with a Mac from the shelf, it will not be associated with any ID. If you install the OS from a stick you prepared it will not be associated with any ID. Although that is the first thing Apple urges you to do once you start up the computer. Anyone can skip this process never associating the machine with any ID.
This OS can be updated just fine to the maximum version of the OS installed.
And since this is an OS that came with the machine as bought, Apple not wanting money for this I fail to see any legal problem arising from that. Apple wants to sell you the hardware, which you bought, and to push content for which you have to pay again. The OS being just a free giveaway enabling you to get in.
Technically, you can not update non-professional apps from the AppStore since that indeed requires giving the credentials for the AppleID that was used to download them. But again technically, you will most probably be able to update most of the professional (or real applications/programs) software installed on that system until said software is payware and hits the paid-upgrade threshold. Free/libre professional software can be amazing too and you will never have to pay for that, and you can update and upgrade those titles as freely and happily as you like. The OP never stated what kind of amazing software was meant. Legal reinstallation might just be a hassle etc. Reselling should be possible and legal everywhere, even with software that's now defined 'as a service' (meaning: 'just for rent').
Legally you might have a problem with that subspecies of payware professional software. If the original owner sold you the software license as well you should be fine, but certain jurisdictions enforce the unethical business practices of forbidding just that. You may break the law in using that kind of software and if applicable you should strive to improve the legal situation for that by lobbying and urging to change that stupid law.
Back on the Topic of your Actual Question
Legally, technically and ethically: yes, you can delete the files of other users/previous owners and you should delete them; but ethically you should not 'access' these files. Instead you should probably follow Steve's advice or just go to the folder
/Users/ and select these folders there:
admin (admin) from old corporate IT department
old user no. 1
old user no. 2
and delete them unseen. That should be possible either via Steve's method or just by dragging the folders there to the Trash.
And finally, I agree that a complete wipe and clean install is probably by far the best option for even more reasons than already stated here.