Stephen,
You raise some interesting issues.
Let me be upfront – I have no experience of an outboard on the aft end of an Atalanta.
Balance – a small outboard on the aft end is not going to be much of a trim issue unless you also have an 80 kg person perched on the aft end operating the outboard.
The propeller need to be well immersed – the last thing you want is the propeller to come out of the water when the boat porpoises in strong wash or swell – so long shaft essential.
But I suppose my main concern is practicability. One person is on the aft deck operating the engine; another person is on the foredeck ready to anchor or pick up a mooring; a further person is (probably) in the cockpit ready to deal with sails. Co-ordinating that lot will be interesting – not impossible, but interesting!!
Good luck!
Mike