Tips for hiring a freelancer?

Sorry for another noon thread. Looking to hire/purchase a custom made niche review/article site running on WP. (Please no offers here).

Any experienced members give me any tips on not getting royally screwed?

Also logistically, how does a site transfer like this work. Is the directory just passed over or is something more technical required?

How do you maintain security? It seems like it could be really easy to add code (or just alter affiliate codes) to hijack the site if all code is not reviewed in total. Any tips could help out, guys. Don't want to burn a large investment due to ignorance.