Northern Brewer reviews are almost all 5-star; the DR is next on my list of extract kits to buy when my fermenters are empty/ready, along with the White House Honey Porter
I've done the NB AG kit and then got what I needed locally and all have come out outstanding. I've used 05 yeast on all and would not change a thing. My friends beg me for this beer so I try to make it 4-5 times a year.
I would assume the extract would be totally fine so long as you weren't storing it in a scorching hot location like a garage. Same with the hops. You probably should replace the yeast though. If you're going with liquid yeast, you probably should also make a starter even with new yeast.
You may be surprised how long grain can last if you store it correctly. I've kept crushed grains for up to two months before using - I keep them in the sealed bags in a cool, dry location.
I refrigerate all ingredients until brew day, if you did that I would assume it will still be good.
I brewed that kit in early August, turned out great! I used US-05 (2 packs, re-hydrated) and kept it @68 with a ferm chamber.
I will be re-ordering the ingredients soon (saves about $4), I was really happy with this kit.