Well - Your PC seems to be really poor if You cannot have more than 30 fps then (CS1.5)... How amny bots You are playing with? They are playing stable about 50 fps. I have just tested - they do buy weapons on CS1.5 It may happen they don't buy any weapon on maps like awp_xxx aim_xxx etc, because usually on such maps they should find the weapon somewhere on the ground (or the weapon is given at spawn). Try the latest
beta to see if it works better or not.