I buy used and am able to ride VERY nice bikes (IMO) as a result that I could not otherwise even look at. Let someone take that first 2-4 years worth of depreciation.
Regarding the accessories: I think they are way overrated. This is riding, not posing we are talking about. Get the right bike first. Suffer through a couple of months with a thrift store helmet, pick up a sette pump for $8.00 at pricepoint. Get a $5 allen key set and a $3.00 set of aftermarket tire levers and patch kit. Then get out a ride that nice bike. That is what it is about. I have never heard of someone saying that they wish they had spent less on their bike and more on their accessories.
Older 105 equipped bikes with down tube shifters can be had for under $250 and are still very fast. Remember, STI does not make the bike fast.