Thanks for the tips.
I like the idea of a 'bare metal' BBC Basic computer so I started with the RISC OS Pico variety.
The system boots into basic in under 4 seconds so that is about as fast as the late C64
I have put the Pi A+ inside the case of a Commodore C16 of which the TED and CPU were defunct...
So far I have connected the original C16 keyboard matrix to 16 GPIO pins of the Pi and started dabbling with the GPIO commands and first steps are working, I have enabled pull-up on the inputs and can reliably read out the matrix now.
Next step is to convert the code to some assembler and make it a autostarting module.
BTW, I have found out that *SCSI starts up the filesystem for accessing a USB stick but is it possible to access a second partition on the internal SD card ? Since the A+ only has a single USB port I attached a USB hub for now to use the USB keyboard and a USB stick for storage. If the internal keyboard starts working then it is not such a problem anymore. Still, would be nice to use some of those extra GB's on the sdcard.