humm I just learn the numbers for the wepaons I want.. so I just hit B, 2, 1.. and for ammo there is a quick buy button without bringing up the buy menu.. those are , . for main, and secondary ammo..
I think there is a way though to bind keys to buy your weapon of choice.. and all the ammo with one hit.. just never needed it.. as I normaly used about only 3 weapons.. and body armor.. some times.. also don't use the graphical.. I use the text menu to buy from..
or at least I use to.. havne't played in a year or so..