With regards of buying licenses to Acrobat, you can buy a perpetual license or a subscription. A subscription is paid for by the month with a lower monthly price if you commit to a year long subscription. The perpetual license is a one-time purchase but it does have significant limitations you should be aware of. The license is for the current version of the product running on platform (i.e., MacOS or Windows) you choose and the currently-supported OS version and hardware. Assuming you keep your computer with the same OS version and hardware for a “lifetime” and you don't expect new features or bug fixes after the next Acrobat version comes out, you'll be fine.
I'm not trying to push you either way, but thought that you should understand the issues involved since you invoked the word “lifetime.”
- Dov