Your ingredients look good, but since I just do partials I'm going to convert your recipe to DME for my understanding first.
Your 18 lbs of malt equal 10.8 lbs DME.
I rarely ever use over 6 lbs of malt to 4.5AAs, but I remember using 8 lbs once. Looking at the AAs % I would brew it and be satisfied with the bitterness.
Are you looking at 5 gals at the finish? I would think your OG would be higher than 58, maybe more closer to 70-75 depending on your efficiency. A pretty heavy brew for a helles, closer to a fest beer.
I don't like a lot of bitterness to my brews although I've had my share of Helles in Germany and just went with what I was served at a local fest. Never disappointed.
If you can you should download Beersmith, or is it Brewsmith, and run your ingredients through that.
It would be better able to tell you more than just my guesses.
