I bought mine in 2010 for 10 euro with paypal (wasn't $10 at the time, they were a euro only shop).
Anyways, you have to use the migrate account thing if you didn't have email to sign in with, the other guy linked that. I migrated mine a few years ago I think? I don't think there's a time limit for that
After you migrate to the mojang account, you should be set for the Java edition. Which is the one with most mods.
If you want the microsoft edition, there's supposed to be a way to get that too, but I had difficulties last time I tried.
Edit: Whoops! Looks like I'm a month past the deadline to get my free windows 10 version
https://www.minecraft.net/en-us/article/ancient-java-players--you-attention-please-