Purchase your accessories at the same time as the bike. The margin on accessories is much higher than on bikes so you should negotiate for free extra bits rather than cash off the bike.
Fenders will keep you clean rather than dry, esp if they are full-length bolt-on style. Road runoff is a mixture of oil, diesel, rubber dust and dog doo. I dont want this sprayed all over the back of my waterproof jacket.

Everyone is aware of helmets and safety, whichever side of the debate you stand. You should also consider cycling mitts/gloves as a safety issue. If you come off you are far more lightly to injure your hands than your head. With gloves you just get a few bruises and can ride to work. Without gloves you skin your hands, visit ER and take about 2-4 weeks to recover use of your hand.

Spare inner tubes are useful. I carry one, keep one at work and one at home. You can repair them at your leisure rather than by the roadside.
The most useful item of clothing is a lightweight, bright, cheap windproof (non-waterproof) with some reflective trim.