For interest have you tried the sleeper? How well did you sleep? I am unlucky in the sense I struggle to sleep on transport, sometimes the only on in the cabin awake on a long haul flight.
I slept pretty well on the sleeper, as I was lying down and it leaves fairly late, so after a bit of food in the lounge car its time for bed, I woke up about 5:30 in the morning to go to the toilet. (and realise we where still at crewe as the knitting of th WCML died)
Certainly better than a Red- Eye express at 4am. Just a shame if you are going single there arent reasonably priced single accomodation (I think japanese pod hotel stylie, however others like the idea of airline style solutions)
With regards to MK4 sleeper conversions, the carriage is already strong enough as it is, so it would just be interior walls and fittings needed. I imagine you could do something reasonable out of wood and plasterboard if you wanted, just line up the compartments with a window and add a decent set of blinds/curtains. Giving better views during the day, and if done right a ncier environment