Some of my personal preferences
-Pedals: On my commuter I have Shimano A530 MTB pedals. They are SPD, so the shoes are walkable. They are dual sided as when you commute you are frequently in and out and don't want to fiddle around with the right side. They have a cage so you can make short trips with street shoes relatively well.
-Shoes: I have the Pearl Izumi X-Alp shoes. They have a running shoe look which is nice when you are off the bike. They are not super heavy/stiff, but enough so I don't feel like I am flopping around. Since I mostly use these for my commute and off road ride which is less intense, I don't need the straps. I put a pair of elastic laces in so they slip on and off quickly. Nice for switching between work shoes and bike shoes.
Helmet: I use a Giro general purpose helmet. It would be considered a MTB helmet really, but the visor comes off. When riding in the drops the visor can be an issue, so I don't even use it. Often not a ton of difference between a MTB helmet and a road helmet at the sub $75 mark
Bag: Personal Preference. I say order one online for far cheaper than you would pay local unless they give you a solid deal when you buy the bike. Medium or large size will probably work, its basically when I have on my CX commuter.
Lights: There are tons of opinions on these. I like the Cygolite Expilion series I have on my setup for the front. I have the 350 lumen model which is plenty for my use. I also keep a Knog Boomer USB on the front in flashing while the Expilion is steady on the helmet. I have an XLC tail light as well. What you will need depends on where you ride, the lighting conditions, and your comfort level.
-Glasses: In general, I like to wear glasses when I ride. I do clear when it is dark/cloudy, and brown photochromic when it is sunny. These are important for sun, comfort (wind), and safety.
-Pant Straps: My commute is short and I wear jeans most days. So given that, I wear my work clothes in. I use reflective velcro straps which are good for visibility as well as keeping the pants out of the chain/crank
Depending on any number of other factors, you may need different or additional things. Backpacks or bike packs, racks, fenders, locks, rain gear (if you will do it all weather) may be needed. General items such as patch kit, spare tube(s), pump/CO2 (or both), computer/GPS, cell phone mount (if you so choose), riding specific clothing (jacket, jersey, shorts), tools, etc. may be in order if you don't have them already. How much how little you spend is up to you.