Actually, if you were going the freeware route, I would go LibreOffice, which is very much like OpenOffice, but is updated much more often.....but....
First, if Excel is very important to you, you may have some issues with the freeware versions...Close, but no cigar as they say.
And, despite lots of work and some very, very good effort, there are times that things come out a bit "different" shall we say, if you try to make either OpenOffice and LibreOffice "play nice" with MS programs. For the most part, no real issue, but at times....and it always seems the worst time....
For the most part, not a big deal, except when it becomes a big deal...and as much as I do not like to steer any $$$ toward Microsoft, for "not so tech savvy" people who "just want to get on the computer and do it," MS is the best choice (man it hurts to say that).
But, by all means, you could try LibreOffice or OpenOffice and see how it works for you. You may love it -- but again -- if you are a real Excel monkey, hmmm...
There are several ways to save big $$$ on MS Office, but you sorta need to "know" how and that is something you pretty much have to figure out yourself -- at least I am not going to push you any direction...
However, the suggestion above about a work discount is a good easy way -- also some community colleges and universities have agreements with MS for discount software for students and/or faculty and staff, but not as many as ten years ago -- apparently, at this time, if you qualify, the software is FREE.
So, give the freeware suites a shot. There is a good chance they will work well for you. If not, you can always push some $$$ toward MS>