What microprocessor for experimentation?
I agree that ARM is the way to go for 32-bit microcontrollers.  ARM is ubiquitous and its assembly language can be used across a broad range of microcontroller families.  ARM also has good support from the GCC toolchain.  The ARM7TDMI chip architecture has dominated the 32-bit mcu space the last 5 years and the ARM Cortex-M3 is the emerging replacement.  The  Cortex-M3 does have a Harvard architecture (separate instruction and data address spaces), but I don't feel that's a limitation.  

Micromint has a solid reputation, and they offer a Cortex-M3 board with configurable options for a decent price.  However, if you really need DIP configuration, I've had success with the mbed

Now, the next thing is languages.  You mentioned FORTH.  I also recommend Python-on-a-Chip and eLua as powerful, easy to learn languages that work on this size target platform.  eLua is more fully developed but has larger resource requirements than Python-on-a-Chip.  Full disclosure: I'm the author of the PyMite VM used in Python-on-a-Chip.  So, if your goal is to make your own language, I fully understand the joy of that exercise.