I must say that I was quite surprised that portenos (esp 18-40) dress very casual. This is contrary to what everyone told me before my arrival in Argentina.
Everyone advised me that portenos are really apperance orientated and dress really well and that nobody wears shorts and sandals and all that.
IT IS NOT TRUE. Do not listen to this advice.
I went down packed with dress pants, black shoes, dress shirts with no shorts and no sandals.
Once I arrived (early November) I discovered that most guys my age (30's) dressed in either jeans, tshirts and/or shorts, actually lots of local guys wear shorts in BA. (and not to just work out).
I found the reality is that most Portenos dress and look like most other people in the other cities I have been to so do not sweat the details - you can wear practically anything and you will not stand out excepting fancy restaurants and business meetings.
I walked all the major hoods in BA in 1 week and found that the reputation of the super stylish, great looking, great bodied Porteno to be more a PR, Travel Magazine fiction then reality so potential visitors do not need to be worried about what to bring to BA.
Actually do not bring anything as the shopping is great and prices are cheap to reasonable for the quality with the vast majority of stuff made locally.
If going out for a drink , to a boliche (club), or just to have dinner..PLEASE DRESS UP! No sneakers, no "gym" clothes..no matter how expensive. Make an effort to look good because portenos dress up and look amazing (most of them..quite few exceptions, tough).
If you need to get a top, a black dress (always a life-saver), or something "smart" just pop into a local Zara (FLORIDA, 651 BUENOS AIRES) or C&A (also in Florida). These two stores offer inexpensive quick fixes.