dull coat ,brushes,reference pictures,art chalks,various dull paints. alot of weathering can be accomplished by varying the sheen of your paint for a car that has been well cared for for twenty years somewhere between gloss and semi gloss should do it .as for the chalks it's cheaper to get them from a craft store than a hobby store ,and with these you basically just grind some off the chalk apply with a dry brush but you should plan on sealing it with some sort of clear coat so it doesn't wipe off. as for colors it just depends, this is where the reference comes in , some time the weathering calls for contrasting colors like tan mud on a white car but in your example maybe adding some chalk close to the body color mixed with dark gray or a light brown would give you that not so new look around the wheel wells and window posts or anywhere else that dirt will accumulate. practice is the main thing and there are dozens of ways to skin this cat. hope this is enough to point you in a direction that is helpful.mostly though experiment and use what works for you good luck