Be highly sceptical of web-sites selling MS licences that are far cheaper than retail cost. They might well be genuine licences but they almost certainly won’t be allowed to be resold
The site link I deleted was “legitimate” as far is I could tell but seemingly resells from illegitimate sources so it’s pot luck as to whether you get a ‘good’ licence or not. It looks like they also resell a key multiple times.
Though tbh I’d go for Office 2011 (Mac) or 2013 (win) as a one-off install. It hasn’t gotten much better.
This is really an “it depends” statement. If like 95% of users you “need” Office to write a letter every couple of months and add up the scores in your weekly Zoom ‘pub’ quiz then Office 2019 / O365 isn’t going to be bringing much to the table over Office 95.
If on the other hand you’re using it in anger then it’s come on leaps and bounds in recent years. You get versioning via OneDrive, so never have to press “Save” again. Collaboration and document sharing is light years ahead of previous versions. Integration with other apps, Active Directory backed security…
TBH, most home users who “need” MS Office probably don’t. There’s a reason the core apps are free online now, no-one’s going to pay several hundred quid for a hugely complex software suite that they barely use (and indeed, barely know how to use).