It is generally not prohibited to buy an old website with external links and redirect it to your own site, but it can have consequences and should be done carefully.
Firstly, the external links may not be relevant to your own site's content, and redirecting them could result in a negative user experience or even harm your search engine rankings. Additionally, the previous owner of the site may have agreements or contracts with the external sites that could be violated by redirecting the links.
Before proceeding with such a purchase, it is important to carefully analyze the potential risks and benefits and consult with legal and SEO experts and the best digital marketing agency. It is also crucial to ensure that the redirect is done properly to avoid any technical issues or penalties from search engines.
