Q: Is there a development kit for Pascal/Basic/Forth etc
No. The only development kits are based on C. There is no theoretical reason why someone could not create such a kit provided they can create or adapt a compiler that produces Z80 code. However, in practise it's probably easier to teach yourself C.

Q: Is there a manual for the functions
Sort of, see system calls. This is a machine translation from Japanese to English of the manual that was supplied with the DS2 SDK. It requires future manual editing, but is of some use. Also note that not every function that appears in the header files for the SDK is documented in the manual.

Q: How do I run the emulator
See emulator FAQ

Q: Where can I obtain the REX.bin firmware?
Here!

Q: Can I destroy my rex by messing with the firmware?
Probably not. The Rex firmware has a crucial 8k area that handles downloads of firmware. So any normal download cannot overwrite this 8k space. Hence the Rex will be restorable by downloading the original Rex.bin again.

Note however that C. Harris now believes it is possible to overwrite this 8k but only by a separate method.

Q: Where can I obtain the DS2.SDK?
Here..
or Here!