We totally get that the word "invest" means different things to different people, whether it's spending $50 on one item or $1,000. However, we think that if you're going to get a new leather bag that'll actually last you, you should be ready to drop at least $100. Brands aren't terribly important, but quality definitely is: You want a bag made of leather that'll look better with wear, and not worse.
So, your first piece of evidence should be which types of vintage designer bags you love the look of (Coach, Louis Vuitton, and Dooney & Burke are all solid choices). In terms of shape and size, you'll want something that's big enough to actually fit your essentials, plus room for some extras, but not so big that it weighs you down. We'd also avoid getting a bag with too much emphasis on the hardware — off-trend studs, spikes, rivets, and embellishments are the quickest way to date a bag.
Coach Stewardess Bag, $298, available at Coach.