When I started in private practice as a lawyer, I bought three suits and one blue blazer. Given your needs, I'd consider a similar approach. The blazer was worn on Mondays and Fridays with the suits rotated in between. I made that wardrobe last for several years. You'll be surprised, but a decent quality suit (not the most expensive, necessarily) will last for several years even worn once per week when cared for properly. By the way, proper care may mean less dry cleaning than you might think--dry cleaning can age a suit when many times the wearer is simply looking for a pressing.

I'd take a blazer and a suit for each trip, wearing the blazer on the plane. Aircraft A/C can sometimes be too strong, and if you need to slip something on to warm up, a blazer is as good as a sweater. Again, you save some room inside the bag, and if you're on a three-day interview, follow my private practice rotation: blazer first and last days, suit for the middle day.

As boring as it might seem, I'd consider solid color suits, perhaps grey, olive, and tan, depending on your taste--that way you can easily match the slacks to the blazer, thus needing to take only one more pair of slacks. With solid suits, you could consider a pair of patterned slacks to wear with the blazer if you're looking to break up the monotony of solid jackets and pants. That gives you three looks for three days. Solid suits also make pairing shirts easier since you can wear either solid or patterned shirts. Three shirts and two or three ties. Throw in a casual shirt or a polo shirt for your evenings and off hours.

One pair of loafers that can match with a suit (as the world's gotten more casual, simply wearing shoes that can be shined is probably sufficiently dressy to get you by as opposed to the old lace-up dress shoes once consider more "serious," and loafers will be more flexible for your off hours). I'd consider buying a pair of shoes either built for comfort (Rockports, for example, are very comfortable and in some instances, waterproof, and thus able to survive an accident better) or for conservative style with a reputation for good wear (Allen Edmonds, for instance). Comfort shoes will be cheaper, style shoes more expensive (although the right type shouldn't be uncomfortable) but you should take care of those shoes as if you were in the military--shoe trees and treated with leather conditioner and shined after every trip.

The oldest rule in packing--put everything you think you need to take on your bed. And then remove 50%. It really is possible to get by with less.

In regards to the bag you use: spend your money on your shoes and your clothes first because that's what your interviewers will see. They won't see the bag you bring all that stuff in. If you have money left over, there are plenty of expensive bags you'll see touted here and across the internet, but the truth is, you buy a bag to satisfy yourself--they all enclose roughly the same space. If you think you need a Briggs & Riley (or Tom Bihn or Red Oxx, which is my personal favorite), that's fine; if you think you can get away with a bag from Target, that's fine, too. Since you'll likely go carryon only, free from the rigors of airport baggage handlers, you'll find you can make almost any bag last a long time.

Just to reinforce what you probably already know, your interviewers will have the most lasting impression of what your personality and intelligence bring to the interview, the clothing will help make the first impression and perhaps give you a boost of confidence, and the bag you take will make no difference to them at all.