So... I don't have a proper Atari power supply yet but I injected some DC at the diode bridge, which showed me that at least one machine boots to Memo Pad. The two mechanical keyboards are reasonable. One is a "B Key", the other looks exactly like it but is branded Cherry and has a few keys in extra colors. Both use Cherry M8 Open switches, both have full sets of low profile keycaps for MX-compatible stems that are very good once cleaned. (If I wanted to use a 400 I could make a very nice keyboard with some Kailh Choc low profile switches.) The cases are a problem, there is one good top piece. There is one bottom half with just one strip of vent plastic broken that I think I can glue invisibly. The other good bottom half would be perfect but it's the one someone drilled two holes in to add two RCA jacks for a DIY composite video circuit.
I think it's very likely I will sell the two keyboards and any left-over internal bits and put together a 400 system using the membrane keyboard, just to keep an original 400. To use I bought an 800, I have Incognito and Sophia 2 on the way! I'll be designing/printing a replacement panel, definitely will not be cutting any holes in the 800.