I use 2.5 gallon kegs for my batches. It works perfectly for my purposes, since that's my typical batch size. Adventures in Homebrewing runs pretty good specials every few months, so if you can wait I'd just watch their sales and buy a bunch once they're available at a good price. While they are typically less expensive than a normal 5-gallon corny, it's only by 10-20 bucks, tops. So on a per-gallon basis, 5-gallon kegs are quite a bit cheaper.
You could keg your meads as well. The only issue I see is that if you keep the keg under pressure, then the mead will end up becoming carbonated. I think you can avoid this by using a different gas (like nitrogen or argon) to push the mead, but I don't have any personal experience with that.