These items will give u very high amounts of AP late and early, I also included Will of The Ancients for the spell vamp to heal, which also gives the aura to your nearby allies. The Abyssal Scepter also will give a magic reist debuff to nearby enemy champions, also incresing your magic resist as well. Rylais will give you AP and Health, and allows the use of your abilities to slow enemy champions. Since Rumble basically has spammable abilites, your can easily slow down your target in no time.

My runes are set up specially for AP rumble. You will be able to dish out extremely high amounts of damage, even though you are not tanky.

For Rumble I would definitely chose exhaust and ghost. Ghost can be used with Rumble's Scrap Sheild, to make it easier to chase or flee. Exhaust can help you with various fights.

Rumble is a very good team player if you can grab his Ulti and Flamespitter at right times. Try to make your ulti hit all of the enemy champions in team fights.