There is no way to use Ahrefs tool for free in a lifetime. Their trial costs $7 for 7days. That is the cheapest price I have ever seen.
Ahrefs is really useful for you to research keywords. If you think you need other free alternatives to Ahrefs, you can read How To Do Keyword Research + 20++ FAQ. You may found good alternatives to Ahrefs at a cheaper price. You may also consider using keywordtool.io which is free to use with some limitations. You may also consider using SEMRush which has a free trial for 30 days, please find about SEMRush on the link given above.
You can use Ahrefs to analyze many domain and backlinks.
If you build a website that generates an income (or you’re in the process of doing so) – than tools like Ahrefs will pay for themselves with the amount of value that they’ll provide.
The value of Ahrefs will be paid off over time if you use this research tool correctly.