A license for Office 365 can either be bought online from Microsoft or from a computer store. Unlike 2016/2019 which are one-off purchases of a perpetual licence, 365 comes on a subscription basis with annual payments. I believe any Office installation later than 2010 will simply be replaced by the subscription license and reverting would require you to install the old version from scratch.
The insider program is selected when setting up and choosing update frequency. If it is to be your main copy of Office you will need to bear in mind that your work will not be immediately transferable to others and, since it is a beta release scheme, the weekly updates you receive may not be entirely stable, though I have experienced no difficulty to date.
It took me a certain amount of soul-searching before I decided to commit and purchase an additional license. For me, it is pretty close to how spreadsheets should always have worked, though I can see it might come as a bit of a shock to some!
There are also some other interesting functions available, FILTER for example.
= VLOOKUP( fKey, table, {1,2}, 0 )
would return a record from a two-column table but, then again, so would
= FILTER( table, key=fKey,"missing")
The latter would also return multiple records if they matched.
Similarly
= SUMIFS( value, key, fKey )
returns the sum of any matches but so would
= SUM( FILTER( value, key=fKey, 0) )