To answer the original question, which was "Which of the following Hornby Class 56 models could I use for a donor with no modifications (I may have missed some models)?"
The answer, as has been alluded to, is, the chassis off any model ending in xs. Because of the strange/non standard Hornby chassis on the non-sound models with common negative LED's, there isn't an easy solution
Whilst the 56 chassis comes up regularly on ebay, I've never seen a 21 pin sound one.
They are on a board which is glued into the cab.
There is an article on page 78 of Model Rail magazine from June 2012 which explains various modifications.
And it will make your LED's go "pop"
No you can't. The class 60 cabs are/were available as a spare.
The connectors have nothing to do with the issue. The connectors were replaced from R2750 onwards because the ones fitted to early models were unreliable. The newer style connectors are available currently on ebay and from Peter's spares. An ideal upgrade if you have an early model.
Option 1: Buy an xs model, use the chassis and sell the sound chip and body on.
Option 2: Buy an "8 pin" chassis, remove the circuit boards from the cab ends and solder the LED's the opposite way round. Unless you are an expert solderer it is very difficult to remove these intact, they are 0603 SMD LED's and will need to be replaced. It is important to note that the replacement LED's should be soldered in with the polarity as shown on the circuit boards. In the sound models, Hornby have soldered the LED's into the circuit boards in the xs models the opposite way round (positive to negative, negative to positive) to correct their "issue". make them positive common and enable them to work with the standard 21 pin circuit boards .
Option 3: Buy an 8 pin chassis and modify with a ESU 51968 board (and blanking plate for running on DC).
Perhaps someone has a better suggestion?