It's both for the learning challenge as well as the practical need. I guess I'll just order a couple of buck/boost-ICs, but it would be fun to be able to make something that works in the meantime.
I think you will have the chips home faster than you manage to get something semi-working from the naked 555. Just sayin' ....
So do these dedicated ICs regulate both the frequency and the duty cycle or just the duty cycle?
They regulate duty cycle, the frequency tends to be constant. The amount of energy passed to the inductor for each cycle depends on how long was the output switch open/closed = duty cycle (the area under the voltage curve, etc.). So you don't really need to change the frequency too. The ICs don't do it, because the switching frequency has a large impact on the inductor and capacitor choices in the circuit.
By looking at examples on buck/boost-converters it seems that the output voltage is affected by the duty cycle alone? E.g V_out = V_in*1/(1-D), given that the inductor corresponds to the set switching frequency. Is it that simple?
The voltage depends on the duty cycle, but also the inductor choice, output load, etc. The choice of the inductor is quite complex affair - it doesn't depend only on frequency but also saturation current. Ideally you want an inductor that is driven almost to saturation so that it is not unnecessarily large. However that is no good if your load isn't constant because you don't want to actually drive the inductor into saturation = losses and the circuit is unable to deliver more energy demanded by the load, so poor regulation. So this is quite a complex dance how to pick the right part. Datasheets for the converter ICs will tell you how to choose the inductors and what has to be considered.