So, progress is being made.
I bought a used DE M904E, which was in a worse state than I'd assumed, but it had its stock gears replaced with 13:1 SHS ones (short stroked by two teeth, I'd rather they had been as is) so there's one less thing to buy. I'd spent far more than I wanted to, but the inside of the gearbox was a bit of a shit show, and remaining parts will arrive soon for me to crack on and hopefully get it running ready for the Red Alert Battlesim Lite next weekend.
My attempts with painting / rattle cans continues...