Which type of fireplace should I buy?

If there is an opening you could go for an electric log burner type which uses a fan or you could use a flat one as you are thinking. However, there are many ways of using the space. You could put a display fire in it, a display of flowers/lights/other or if you are feeling really creative, you could get a high quality image of a fire and set it back a tad. Printing these days, especially on some of the banners I have had from eBay are so good you can't tell that they are printed, they look perfect and not pixelated like they used to be and they will print it to size... Personally, these days, I'd go for the latter.