I finally bit the bullet and ordered the Aoshima Thunderbird 3 kit from Hobby Link Japan. It should get here in a couple of weeks.
From what I've been able to tell it's about 25cm long, which makes it a bit longer than the JR21 toy version I have as a restoration project. So not only is this larger and better detailed, but the shape is also accurate.
It also comes with three optional docking rings (the white collar) and quite a comprehensive sheet of decals.
So, as far as accurate Thunderbirds models as normal plastic model kits go we have:
Aoshima 1/144 Thunderbird 1
Aoshima 1/350 Thunderbird 1
Aoshima 1/350 Thunderbird 2 (can be used with Aoshima and F-Toys 1/350 TB1)
Aoshima 1350 Thunderbird 3
Imai 1/600(ish) Thunderbird 5 (oldest of these good kits, but it's pretty much an accurate model... certainly the most accurate of the original 1960s models)
Aoshima 1/72 The Mole
Aoshima 1/32 FAB1
Aoshima 1/350 Fireflash Atomic Airliner
We still need a good scale Thunderbird 4. I have the Bandai display version (around 1/72 scale) but it has quite a few shape problems and a terrible interior. We still need good plastic kits of the Firefly, Recovery Vehicles (master and remote control pair) as well as machines like the Domo, Excavator, Elevator Cars, etc...
Maybe one day.
So, here's to watching for the postman!