in plus to what Mylenium said, it's better to get the full CC suite since the price is almost closed between one app and all apps and you will get a benefit from all the other adobe apps.

Adobe only offers subscriptions these days. You're ten years too late for a "buy once, use forever". It's a moot point, though. AE was never even sold separately and required to buy a whole suite, so one way or the other you had to spend more money than you might have wanted...

Mylenium

You must first download the Creative Cloud desktop application, after installing it and logging in with your Adobe ID, from there you can buy a monthly subscription of Adobe applications.

Creative Cloud Plans & Pricing