detailing the out side/inside of a dio

[stepside83]

im about to get the 2 stall grage with office and bath room from smbc just to start in to the dio world, and i wanted to make it look liked its been used when its done being built, i dont want it to look like it was justbought, so how could i make the floor look like concrete? and the out side a little weathered? and maybe an oil spot or 2 on the floor? i dont have an air brush yet going to get one what would be a good one to get for not alot of money? thanks