I think good links should have the following metrics - TF>25, DA/PA>35-40, as these would be the primary filter when picking a good link for your site.
When vetting the vendors, you may ask for a sample or request them to provide you with a list of their site's metrics, and if they meet the minimum, that could definitely be helpful to your site as a whole.
You can then additionally filter the sites based on whether there are any spam links that could be present - spam links will not be good for your site.