Most development can be done with an emulator (see above post for emulator link) and cross-assembler. For doing testing on actual hardware the silver/black console is desired because it will recognize software cartridges with ROM only (no TI GROMs needed as with the beige console).
BTW, Atariage is the forum for discussion and questions related to all modern 99/4A development (hardware and software) - which is still taking place. There will be someone from that forum who can answer almost any technical detail related to the 99/4A or the 9900. You would like find several users on that forum who would jump at the chance to do some testing of any 9900 port. If you want to move forward with this, PM me and I can recommend some of the power users.
I will be receiving some additional 99/4A hardware this week, which I believe includes some full systems - console + PEB (Peripheral Expansion Box) w/memory expansion and floppy drives. There are also some newer peripherals which integrate the 32K memory expansion, floppy emulated on compact flash, as well as serial port. With this much smaller peripheral you don’t need the PEB, which is good as the PEB is quite large.
There is also a board available that allows a USB (including wireless) keyboard to be used instead of the built-in keyboard. With this, you can then shove the console+peripheral board to the side and use a KVM switch.
The 99/4A outputs composite video at 256x192 resolution, so you’ll need some kind of line doubler box to use it with a VGA monitor. Another option is to install a 3rd party replacement, the F18A. The original version outputs analog VGA, but there is a new version coming that outputs HDMI.
If you can support page RAM, there are several boards with (corresponding psuedo-standards) that support more than 32K of RAM. Two of the most popular are the Horizon ramdisk and SupeRAMs boards. Most of the emulators support these boards, most development can be done without the actual hardware.
One of the best sources of technical information on the TI 99/4A has been compiled by Thierry Nouspikel, and can be found on his TI-99-4A Tech Pages. There is detailed here info on all of the 99/4A chips and peripherals, including several that Thierry designed himself. He is truly a 99/4A savant, who started is 99/4A activities while he was a pre-med student. He is currently doing DNA-related research. You will be amazed at the depth and breadth of his 99/4A information.
If you are interested in doing a 99/4A port of your python project, I will donate the necessary 99/4A hardware needed for testing after you have everything working on the emulator. This would include: console, peripherals, keyboard adapter, line doubler/F18A. With this you could then use a KVM if that is more convenient.