Get the specifications of the current server, and see if another with the same characteristics can be identified at a lower cost.
Check the platform used and the related plugins/theme. Ensure you know the technology and that plugins have been updated.
Usually, you can do this by looking at the source page of the website.
Ask also the seller for the screenshots of the plugin and theme section of the website.
Some paid plugins and themes might not be included in the P&L worksheet. It’s better to double-check.
Check the price of all plugins and the theme installed.
Check the MX of the website domain to see if they have a bad reputation, which may indicate that this domain has been used to send spams.
It can be done with Spamhaus or a similar website.
Knowing that the website contains custom code will enable you to Identify potential maintenance issues. Therefore, you need to ask the seller how the website has been built and if plugins and themes have been modified manually.