Simply put, the search and display networks are completely different beasts and need to be treated as such. It will save you a lot of time and headache to separate these two networks, even if they are lumped together when you inherit the account.
To quickly recap, the search network refers to ads that display based on keywords entered directly into a search engine. You know that the user typed in this keyword in their search, which in turn triggered your ad to show up in the sponsored results area. You then also know that the user is searching, actively, for information regarding your client’s keyword. How far along in the process they might be is not as clear. They might be simply browsing, gathering information, or just wasting time. They could be interested in buying your product or service. The point is that they are actively involved in some facet of the sales cycle.
Conversely, the display network takes keywords from your ad groups and pairs them with websites that are also using these keywords. There are overarching themes that the various search engines use for this process, and it is often hit-and-miss unless you have a well-structured account. Regarding the content network, users are first and foremost interested in the content of the website they are viewing. Your content ad is initially nothing more than an afterthought in the minds of the user. For a user to click on an ad, and more important, convert, you must adapt your entire account structure to separate the two networks.
The first important reason to separate the display and search networks is that you need to create tightly themed ad groups with relevant keywords for the content network. If you want to get traffic to your site and improve the CTR, you can use more general keywords that will trigger matches in the content network. However, if you want conversions, you must be sure to keep your ad groups manageable with close-knit keywords. Ad relevance is the key here, and your keywords are the foundation.
Moving on, you need to look at the display network bids so you can adjust them accordingly. Typically, the bids for the display network can be much, much lower than the search network. In fact, if you just leave them the same as the search network, you run the (likely) risk of far, far too much money spent in the content network. AdWords always gives you estimates you can use as a baseline. Then you can adjust accordingly.
Further, your display network ad texts cannot be mirror images of the search network ads. It’s fairly obvious why: the user intent is completely different. In the display network, you have users who are not (at least initially) interested in buying anything. They are just reading some content on the Web. Keep that in mind—it’s very important. So change your ad texts to reflect this. Align your ad texts with your tightly themed keywords within the ad group. Depending on your ads and keywords, you may also want to try out different landing pages as well for content network ads, if your overarching ad group theme demands it.
Long story short, separate your search and display network ad groups. It may take some time, especially if your inherited account doesn’t already have this structure in place. That said, it you will spend less money and get more ROI in the long run.