I think what might be throwing you off is the American Two-row Pale, American Munich (Light), American Wheat malts. This is a partial mash recipe, not extract + specialty grains. Partial mashes are not tricky, they just require that you mash the grain components to convert complex starches to fermentable sugars. There should be threads on doing PMs, i will check after this post and if not, start one.
If you would like to convert to an extract recipe with specialty grains you only need to buy the crystal malt and the amber malt syrup. For quantities to hit the specified target gravity you will need about 7.5 pds of liquid malt extract instead of 6, and i would recommend bumping the crystal malt up to 0.5 pds instead of 0.25 to add a bit more assertive malt complexity since you are not using the other grains. You could replace 0.5 pds of the amber extract with wheat malt extract if you are really particular, but is probably not critical.
As long as you follow the above suggestions, and use the same hops, hop schedule and yeast, your beer should end up being quite similar to the recipe.
As somewhat of an after thought, northdown hops might be tricky to find. I live in a relatively good sized city and have a fantastic supply shop and still have never seen them. If you cannot find northdown, i would recommend finding a hop with similar alpha acid content (maybe challenger?) as the 60 min addition does not contribute much in the way of hop character, but bitterness will matter.
Happy brewing, i will check back to see if you have any follow ups.