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
Q: How do I run the emulator
Q: Where can I obtain the REX.bin firmware?
Q: Can I destroy my rex by messing with the firmware?
Note however that C. Harris now believes it is possible to overwrite this 8k but only by a separate method.