Yes, those are dual-layer formats. You don't need to change anything in ImgBurn for burning dual-layer media. As for the price, click the "get dual-layer media" link I've posted above and you'll have a list of online stores where you can find those Verbatim 2.4x DVD+R DL blanks
Yes, you should select the MDS file as the image to burn. Now, if you want to learn everything about ImgBurn, the Guides section has all the information