Thank you very much for the great answer and sorry for the super late reply. Will definetly use some of your advice going forward but for now have conceived a great way to create heightmaps from hand drawn maps. For those interested its as follows:
Take a photo or scan of your map and import it into an art app or program of your choosing. We use ibis ourselves which does just fine.
When in the new project map out on the map the area that is the highest point and create an easy to reference topographical map on top of your existing map in a new layer. Take every 500 ft for example and make it a new line following curvature of your land. (The smaller that number the more accurate the heightmap can be)
on a new layer: with this new map made create a grayscale gradient effect from top to bottom. Brightest white to darkest black respectively. When reaching each new line on the map layer you made use a nee shade of gray until you reach the black. Then simply smudge or gradient this entire map to make it as smooth or rough as you need. With enough practice this can be as good as real heightmaps. Also for better erosion effects and the like you can hand sculpt after import into unreal or use other programs like Gaea which uses a great blueprint style of map altering that can achieve amazingly realistic effects.