Hi daisy, there are four top questions in a row that can help you out, on the Island of Hawai'i forum, for a start.
You might want to describe what you mean by local (style of food? type of ownership?)and inexpensive (what price range). What do you like on Maui, even though we don't have the same things here, would give us an idea.
Neither Kailua Kona nor South Kohala have a majority of local owned affordable restaurants, compared to Hilo and the small towns on the island, but they both have some. How many restaurants that fit the bill would you need to be happy for this trip?
When you say drive a "bit" to a "nicer" beach, I'm not sure what is OK. Is an hour each way a bit? What if there are no nice beaches right near you? Is that OK?
Have you read the Top Questions on choosing between Kohala and Kona?
When you say Kona, do you mean the town of Kailua Kona? Or do you mean the North and South Kona districts? By Kohala coast, you mean the South Kohala resorts, right? There is more to the coast, but it doesn't have beach to the north.
What type of lodging are you considering, hotel or condo or something else?
There are hundreds of restaurant topics here for you to browse. Just use the search and search by Date instead of Relevance to weed out the old old ones. Restaurants or dining are two keywords that work.