'Isolated' is a rather odd word to use. All of the resorts are self-contained and not walkable to anything, so you'll need taxis to see anything outside your resort. And Langkawi isn't very big.

The Four Seasons is the best resort of Langkawi, especially if what you are after is quiet walks on the beach. (The Datai has a beach but is fundamentally built into the jungle.) Take a look at the FS website and see if there are any promo for '3 for 2' or '6 for 4' etc running. If you book through a Virtuoso agent, you will also get free breakfast, probably a lunch credit and an upgrade on availability, which may also bring it more into your price range. Virtuoso promo are combinable with '3 for 2' and other deal on the FS website.