Are there additional motors and/or sensors that can be used to extend LEGO Boost?

It depends on which app you want to use. My impression is that the Boost app can work with one Boost hub, one Boost motor and one Boost color/distance sensor.

If you use the PoweredUp app, you can use WeDo sensors and WeDo motors as well. But you always have to set the ports explicitely and remember that the external ports on the Boost hub are ports C and D. The WeDo motor only offers "Set power to X" functionality, no "Set power to X and run for Y seconds" or "Go to position" or "Turn for 5 rotations". This is a property of the motor, the Boost motor has all these abilities when using the PoweredUp app.

It seems you cannot use Spike motors or sensors with the PoweredUp app and the Boost hub, or at least I could not. Although you can use the Spike motors reasonably well with the PoweredUp app and the small (Bat Mobile) PoweredUp hub. The only thing that is missing then is the position readout.

I do not own a large or extra large PoweredUp motor and no large PoweredUp hub. Given that using the Spike motor with the Boost hub did not work, but using the the WeDo motor did, I am really unsure about what to expect re. the larger PoweredUp motors. I would expect the large PU hub to behave similarly to the small one.

Lego now sell Boost compatible motors separately

https://shop.lego.com/en-NZ/product/Medium-Linear-Motor-88008

I've tried attaching one of the Powered up motors from the batmobile set (76112) This motor looks identical to me as the WeDo motor (although brickset and bricklink don't list them as identical at the time of this writing)

I could not get it to work, all profiles of the fixed build models complain that the external motor is not attached. The free build profile doesn't have any block that looks like the M-motor and using the boost motor block doesn't work either.

So my guess would be that at the time of this writing, no additional components are currently available for the Boost set...

The other way around, i.e. attaching the Boost motor to the hub from the batmobile, works...

I also purchased LEDs last saturday from an official lego store (had to ask, they had them in the back) they work on the batmobile hub, but I can't get them to work on the boost hub.