PIx - you don't need to swap the SPDs for road versions if you don't feel a need to. I know a few IM who use their MTB shoes for them to save the faff of having different systems on road and MTB bikes. the upside of road shoes and cleats is that they are stiffer so you can put more power through the cranks - MTB shoes are designed softer for walking on.
as for aero bars - your call really. they're not essential but most users will say that they will give up to 2mph extra speed for the same effort at normal bikes speeds if set up properly. 2mph over an IM distance can equate to about 30mins in time saving! and that's for not a lot of investment really. they can take a little getting used to if you're not a confident rider but most people, once they've got used to them, will say they are beneficial. but good setup is important - a poor setup will gain nothing. you will also need to get used to them over a period of time as being in an aero position can be uncomfy on the arms, neck, back and shoulders - but again, in time, you adapt to this.