fall weather isn't bad, and you don't really need too much more stuff until it gets down to under 40 degrees.
Arm warmers are one of the most important things. You can wear your regular summer stuff and just add the armwarmers. Push them down or take them off if it gets too warm. Or you can wear a long sleeve baselayer something out of Polartec or Patagonia capilene, or wool, just a silkweight. Not warm enough with just the long sleeve, add the armwarmers. That's why they're indispensable.
Then, a pair of light tights, knickers, or knee warmers, just the same stuff the shorts are made of, no insulation. Long finger gloves, some light or midweight wool socks, maybe a windbreaker or lightweight vest, again, no insulation necessary. At these temps, I prefer an earband to a balaclava, that's for the colder weather. I just overheat with my head totally covered.
As it gets closer to 40 degrees, you can start adding heavier tights with fleece insulation, a midlayer or heavier weight jersey like the PI Kodiak and Kodiak light, windproof vest with fleece insulation, heavier gloves like PI Cyclones, or my fave, Garneau windstoppers, the balaclava, booties or winter cycling boots. But now we're talking about spending a bunch of money.