Buying a website is not that complicated. Finding a good buy from an honest seller is a lot tougher, and perhaps not that common.
The forums used to offer sites for sale, much cheaper. Now the decent sites end up on the bigger sites, and bring good money often times, often way too much IMVHO. Many people build sites just to sell them, which is fine, but they are good at getting their numbers up there. So take away two things. First your numbers likely wont stay as strong. Second they built that site, and they can do so again. If they built one unlikely they wont build more, or already did. Its not like you bought the only website on that topic on the net.
Let me cover “turnkey” websites first. It has a couple of meanings on the net. Its just like the real world, its already up and running and is fine as it sits. In the web that of course isnt true, especially when it comes to what a great deal of us think of as turnkey sites. Several years ago someone produced a website based on an evergreen topic, lets say stopping smoking, and they sold countless copies. People knew what they were buying but tons had the exact same site. At the time master reseller rights were big and most all of these sites ended up on them. Now PLR sites would have them, know one still does, but PLR isnt growing right now either. Anyway they made a fair number of these in all sort of evergreen topics. So a lot of people started using them, as they had access through their membership. Anyway the same site on the web 100 times is worthless. I mean the thing about a mass produced “turnkey” site is you are going to need to at least rewrite every article, and rename every pic. I have built a site, based off one of these just so I wasnt starting off with a blank page.
You likely already know that the search engines strongly dislike unoriginal content. You should ask and check with the seller. Copy scape is a good tool for that.