Quick follow up.
The smaller Medivac seemingly had no issues and upon investigation, I discovered that the customer had run out of bags, cut a hole in her last one and was emptying / re-using it. I suspect that it was clogged up and causing the machine to overheat when used for a prolonged period. I didn't want to test this since the bag was definitely iffy and just replaced it with a HepaFlo bag and advised her to use those instead.
The larger Medivac had a motor on the verge of failure. The commutator was clinging on for dear life. The motor was a D21004N, so I replaced it with the Numatic Full Replacement Motor Kit (205779). It was a moderately tricky fit, I used the hard plastic adapter supplied with the motor but it was necessary to use the original gaskets, especially the bottom gasket as that lined up with the extra large HEPA filter that this machine comes with. As such, I should have just gone with the DL21104RT replacement motor instead, which would have saved a few ££.
All in all, they're impressive machines and definitely worth maintaining. Customer called me her "savior" as she'd called around a bunch of places who all declined to even look at the machines as they weren't familiar with the make.
Thanks for the advice, MVacs. Spot on about the brushes being a waste of time.