I would recommend the online activity tracker Snoopreport. With it, you can track up to 100 of any public profiles at the same time. This will allow you to get reports on likes, follows, hashtags and interests of followers of the page you are going to buy. Firstly, it will allow you to understand whether the followers are alive and that they are not just bots. And secondly this way you will be able to understand how relevant this audience will be to you after buying a profile, especially if you are going to change its theme.
In general, this tool is also suitable for auditing the activity of your current or potential audience, as well as the audience of competitors. You can use it to select niche hashtags and plan an engaging content strategy.