Upgrade Microsoft word, excel, powerpoint, outlook to 64 bit version, single user

Depends on the version of Microsoft Office you have.

For Microsoft Office 2016 see here (its free): https://docs.microsoft.com/en-us/office/troubleshoot/office-for-mac/office-2016-for-mac-64-bit-upgrade

For earlier versions (2011, or 2008), the only option is to buy a newer version of Microsoft Office. There is no other option.

https://www.microsoft.com/en-us/microsoft-365/buy/compare-all-microsoft-365-products

Alternatively, you can choose to use Apple's free alternatives, Pages, Numbers, Keynote and Mail. Or a free 3rd party option such as Libre Office or Open Office. Do note those free alternatives do not include a mail app to replace Outlook, so you'll need an addional piece of software such as Mozilla Thunderbird.

The decision between rebuying MS Office or a free alternative also depends on your pervious body of work created with Office.

When faced with your situation two years ago, I tried both LibreOffice and Apple's productivity apps. LibreOffice always changed margins on my Word docs, and Apple's Pages did not handle text formatting in the same way as Word.

Not a big deal had I but a handful of Word docs, but I have thousands. Finding no way to batch convert several thousand Word files generated since about 1991, I bit the bullet and bought the free-standing Office 2019. It is quite nice and I've has no issue with it. MUCH better the the Windows version!!

Cheapest as in capital expense (single-purchase), or operating expense (subscription)? Microsoft has retired Office 2016 for Mac from further updates or support, so I would ignore that product. The options presented on one of my previous posts lays out your options and expenses for upgrading to 64-bit Office solutions.