My suggestion would be to simply create a child theme - it would definitely be less work, and you could extend anything that currently exists in the theme you purchased.
Before you do this, I would suggest installing the Theme Authentication Checker (TAC) and run it through the theme you purchased. If there are a lot of errors, then chances are in the future when you update WordPress, their code could indeed break. If that's the case, I'd suggest using their resources and re-code it - better to start off properly, than have to fix it in 6 months when the code isn't fresh.
That said, developing your own themes is always preferable, if you have the ability and knowledge of WordPress best practices.
As for retaining the custom post types, and other aspects of the them you purchased, it totally depends on the developer's style. Your best bet is to load the site into an IDE with site-wide search, and look for
register_post_type and any other syntax that creates the content you want to bring to your custom-made _s theme.
Hope this helps! :)
Tutorial on Child Themes